Declining inhibition elicited in cat lumbar motoneurons by repetitive stimulation of group II muscle afferents

1993 ◽  
Vol 70 (5) ◽  
pp. 1805-1810 ◽  
Author(s):  
J. Lafleur ◽  
D. Zytnicki ◽  
G. Horcholle-Bossavit ◽  
L. Jami
Keyword(s):  
Muscle Afferents ◽  
Repetitive Stimulation ◽  
Excitatory Postsynaptic Potential ◽  
Constant Input ◽  
Rapid Reduction ◽  
Group I ◽  
Constant Strength ◽  
Group I Afferents ◽  
Group Ii ◽  
Stimulation Of

1. The aim of the present experiments was to verify whether group II inputs from gastrocnemius medialis (GM) muscle could elicit declining inhibitions similar to those observed during GM contractions in a variety of lumbar motoneurons of the cat spinal cord. Motoneurons were recorded intracellularly in chloralose- or pentobarbitone-anesthetized preparations during electrical stimulation of GM nerve with repetitive trains. 2. With strengths in the group I range, repetitive stimulation evoked the usual Ia excitation in homonymous motoneurons and excitatory postsynaptic potential (EPSP) amplitudes remained constant throughout the stimulation sequence. In synergic plantaris motoneurons lacking an excitatory connection with Ia afferents from GM, the same stimulation, kept at a constant strength throughout the stimulation sequence, elicited rapidly decreasing inhibitory potentials reminiscent of those evoked by GM contractions. 3. In motoneurons of pretibial flexors, quadriceps, and posterior biceps-semitendinosus, the stimulation strength required to observe declining inhibitions resembling those produced by GM contractions was 4-8 times group I threshold, engaging group II in addition to group I fibers. 4. These results show that input from GM group II plus group I afferents can elicit inhibitory effects in a variety of motoneurons. Such observations support the hypothesis that messages from spindle secondary endings and/or nonspecific muscle receptors activated during contraction might contribute to the widespread inhibition caused by GM contractions. 5. Inasmuch as constant input in group II and group I afferents evoked declining inhibitory potentials, the origin of the decline must be central, which suggests that the rapid reduction of contraction-induced inhibitions also depended on a central mechanism.

Synaptic organization of defined motor-unit types in cat tibialis anterior

1980 ◽  
Vol 43 (6) ◽  
pp. 1631-1644 ◽  
Author(s):  
R. P. Dum ◽  
T. T. Kennedy
Keyword(s):  
Motor Unit ◽  
Tibialis Anterior ◽  
Muscle Afferents ◽  
Medial Gastrocnemius ◽  
Synaptic Organization ◽  
Ia Afferents ◽  
Group I ◽  
Ia Epsp ◽  
Group I Afferents ◽  
Stimulation Of

1. Synaptic potentials were recorded intracellularly in tibialis anterior (TA) motoneurons following stimulation of a descending brain stem pathway, the medial longitudinal fasciculus (MLF), and three segmental inputs, the homonymous and heteronymous group Ia afferents, the group I afferents from the antagonist, and the cutaneous and muscle afferents. Intracellular stimulation of the motoneurons was used to classify them, based on the properties of the innervated muscle units, into types FF, F(int), FR, and S (6, 16). 2. The sum of the monosynaptic EPSP amplitudes resulting from stimulation of homonymous and heteronymous group Ia afferents (summed group Ia EPSP) was inversely related to motoneuron size, as assessed by motoneuron input resistance, and was inversely related to motor-unit tetanic tension. Type-FF, -FR, and -S motoneurons showed significant differences in the mean amplitude of their summed group Ia EPSPs. 3. The amplitudes of disynaptic IPSPs resulting from stimulation of group I afferents in the antagonist muscle also showed an inverse relationship to motoneuron size. The observed relationships between motoneuron size and the monosynaptic group Ia EPSP amplitude or the disynaptic group I IPSP amplitude are compatible with the “size principle” of motor-unit recruitment (26). 4. The amplitudes of the monosynaptic EPSPs evoked in TA motoneurons by stimulation of the MLF were distributed rather randomly among all types of TA motoneurons. A slight tendency of larger monosynaptic EPSPs to occur in motoneurons with larger tetanic tensions was observed. 5. The polysynaptic effects from cutaneous and muscle afferents in sural and gastrocnemius-soleus nerves were frequently excitatory on type-FF motoneurons, but were primarily inhibitory on type-FR and -S motoneurons. Clearly, the polysynaptic cutaneous and muscle inputs and the monosynaptic MLF input onto TA motoneurons show a different pattern of synaptic organization than the group I inputs. 6. In general, the synaptic organization of the TA motor nucleus is similar to that of its extensor antagonist, medial gastrocnemius (MG) (2--5, 7, 8), when analogous neural circuits are compared. This parallel organization suggests a commonality of motor-control systems for both flexor and extensor muscles.

Repetitive monosynaptic activation of motoneurones

1951 ◽  
Vol 138 (893) ◽  
pp. 475-498 ◽  
Keyword(s):  
Repetitive Stimulation ◽  
Monosynaptic Reflex ◽  
Synaptic Potential ◽  
Group I ◽  
Excitatory Action ◽  
Extensor Motoneurones ◽  
Group Ii ◽  
Set Up ◽  
Reflex Discharge ◽  
Stimulation Of

Repetitive monosynaptic activation of motoneurones has been set up by maximum repetitive stimulation of the group I afferent fibres of the nerves to various muscles. By studying the effects produced by stimuli of varying strengths and also by comparing the responses of flexor and extensor motoneurones, it has been possible to allow for complications arising from concomitant stimulation of group II and III fibres in these nerves. Motoneurone responses have been recorded either as impulses discharged along the ventral root or as synaptic potentials electrotonically transmitted thereto. The repetitive synaptic potential conforms to a standard pattern for both flexor and extensor motoneurones. With frequencies over 100/sec. there is a brief initial phase of summation, then decline to a plateau at about the height of the initial single potential. On cessation of stimulation there is an immediate decline to a positive after-potential which bears a close resemblance to that observed after repetitive stimulation of peripheral nerve. The repetitive potential is com­pounded of ‘diphasic’ potentials generated by each successive volley, an initial negative synaptic potential and a later positive after-potential. At high frequencies of stimulation the successive potentials show an initial rapid decline in size to a low value, e. g. to 50% at 400/sec., which is only in part attributable to a decline in the size of the volleys entering the spinal cord. Focal recording has permitted simultaneous comparison of these two declines. The excitability of the motoneurones during and after repetitive activation was tested heterosynaptically. During the synaptic potential plateau the excitability is raised, with periodic variations in phase with the successive volleys, while depression is observed during the positive after-potential. With extensor muscles the reflex discharge of impulses conforms in general with the repetitive synaptic potential curve, the initial summation giving monosynaptic reflex dis­charges to the first two or three volleys at high frequency. It is probable that a motoneurone fires only once in this initial burst. Subsequently, the combination of autogenetic inhibitory action of group II fibres (which may however be negligible), diminished synaptic excitatory action, and depressant action of the accumulated positive after-potentials (subsynaptic depression) may prevent all further discharge. Alternatively, after a brief quiescent interval, a small irregular discharge may be set up monosynaptically by subsequent volleys. With low frequencies of stimulation the second and subsequent volleys may all evoke reflex discharges, but they are always smaller than the initial discharge. Repetitive activation of flexor moto­-neurones differs from extensors in that the autogenetic excitatory action of group II afferent fibres makes both the initial and later bursts of reflex discharge larger and more sustained.

Responses of cortical neurons (areas 3a and 4) to ramp stretch of hindlimb muscles in the baboon

1976 ◽  
Vol 39 (3) ◽  
pp. 484-500 ◽  
Author(s):  
J. Hore ◽  
J. B. Preston ◽  
P. D. Cheney
Keyword(s):  
Cortical Neurons ◽  
Muscle Length ◽  
Muscle Afferents ◽  
Muscle Stretch ◽  
Hindlimb Muscles ◽  
Group I ◽  
Hindlimb Muscle ◽  
Area 3A ◽  
Group Ii Muscle Afferents ◽  
Group Ii

1. A study was made of the response of single cortical units in areas 3a and 4 to electrical stimulation of hindlimb muscle nerves and to ramp stretch of hindlimb muscles in baboons anesthetized with chloralose.2. Stimulation of hindlimb muscle nerves revealed a group I projection primarily to area 3a but with some input into adjacent area. 4. A major group II projection was found in area 4 adjacent to area 3a. A small number of area 3a neurons receive convergence from both group I and group II muscle afferents.3a. On the basis of their response pattern to ramp stretch, units were classified into one of six categories and their cytoarchitectonic location was determined. Units in area 3a had hynamic sensitivities equivalent to that of the primary spindle afferents. Although the discharge of some area 3a neurons also reflected differences in muscle length, most area 3a neurons had low position sensitivities. One unit type in area 3a did not respond to maintained muscle stretch and signaled only velocity of stretch.4. Units in area 4 had position sensitivities equivalent to that of primary and secondary spindle afferents. Although the discharge of some area 4 units reflected different velocities of muscle stretch, these units had dynamic sensitivities similar to those of secondary spindle afferents rather than those of primary afferents. One type of unit in area 4 had no dynamic component to muscle stretch and signaled only muscle length.5. The results demonstrate that there is a transfer of dynamic and position sensitivity from spindle afferents to cortical neurons. Furthermore, data processing has occurred because some units respond only to the steady-state length of muscle, while other units encode only the dynamic phase of stretch. This behavior is different from the responses to ramp stretch of either group I or group II muscle afferents in the baboon.6. The results demonstrate that single units in cerebral cortex can encode the information transmitted to the central nervous system by muscle spindle afferents. The purpose for which this information is used remains undetermined.

Contribution of hind limb flexor muscle afferents to the timing of phase transitions in the cat step cycle

1996 ◽  
Vol 75 (3) ◽  
pp. 1126-1137 ◽  
Author(s):  
G. W. Hiebert ◽  
P. J. Whelan ◽  
A. Prochazka ◽  
K. G. Pearson
Keyword(s):  
Electrical Stimulation ◽  
Stance Phase ◽  
Flexor Muscle ◽  
Step Cycle ◽  
Locomotor Rhythm ◽  
Ia Afferents ◽  
Group I ◽  
Flexor Muscles ◽  
Group Ii ◽  
Stimulation Of

1. In this investigation, we tested the hypothesis that muscle spindle afferents signaling the length of hind-leg flexor muscles are involved in terminating extensor activity and initiating flexion during walking. The hip flexor muscle iliopsoas (IP) and the ankle flexors tibialis anterior (TA) and extensor digitorum longus (EDL) were stretched or vibrated at various phases of the step cycle in spontaneously walking decerebrate cats. Changes in electromyogram amplitude, duration, and timing were then examined. The effects of electrically stimulating group I and II afferents in the nerves to TA and EDL also were examined. 2. Stretch of the individual flexor muscles (IP, TA, or EDL) during the stance phase reduced the duration of extensor activity and promoted the onset of flexor burst activity. The contralateral step cycle also was affected by the stretch, the duration of flexor activity being shortened and extensor activity occurring earlier. Therefore, stretch of the flexor muscles during the stance phase reset the locomotor rhythm to flexion ipsilaterally and extension contralaterally. 3. Results of electrically stimulating the afferents from the TA and EDL muscles suggested that different groups of afferents were responsible for the resetting of the step cycle. Stimulation of the TA nerve reset the locomotor step cycle when the stimulus intensity was in the group II range (2-5 xT). By contrast, stimulation of the EDL nerve generated strong resetting of the step cycle in the range of 1.2-1.4 xT, where primarily the group Ia afferents from the muscle spindles would be activated. 4. Vibration of IP or EDL during stance reduced the duration of the extensor activity by similar amounts to that produced by muscle stretch or by electrical stimulation of EDL at group Ia strengths. This suggests that the group Ia afferents from IP and EDL are capable of resetting the locomotor pattern generator. Vibration of TA did not affect the locomotor rhythm. 5. Stretch of IP or electrical stimulation of TA afferents (5 xT) during the flexion phase did not change the duration of the flexor activity. Stimulation of the EDL nerve at 1.8-5 xT during flexion increased the duration of the flexor activity. In none of our preparations did we observe resetting to extension when the flexor afferents were activated during flexion. 6. We conclude that as the flexor muscles lengthen during the stance phase of gait, their spindle afferents (group Ia afferents for EDL and IP, group II afferents for TA) act to inhibit the spinal center generating extensor activity thus facilitating the initiation of swing.

Absence of effects of contralateral group I muscle afferents on presynaptic inhibition of Ia terminals in humans and cats

2012 ◽  
Vol 108 (4) ◽  
pp. 1176-1185 ◽  
Author(s):  
Rinaldo André Mezzarane ◽  
André Fabio Kohn ◽  
Erika Couto-Roldan ◽  
Lourdes Martinez ◽  
Amira Flores ◽  
...  
Keyword(s):  
Presynaptic Inhibition ◽  
Muscle Afferents ◽  
H Reflex ◽  
Triceps Surae ◽  
Reflex Response ◽  
Current View ◽  
Tendon Vibration ◽  
Vibratory Stimulus ◽  
Group I ◽  
Group I Afferents

Crossed effects from group I afferents on reflex excitability and their mechanisms of action are not yet well understood. The current view is that the influence is weak and takes place indirectly via oligosynaptic pathways. We examined possible contralateral effects from group I afferents on presynaptic inhibition of Ia terminals in humans and cats. In resting and seated human subjects the soleus (SO) H-reflex was conditioned by an electrical stimulus to the ipsilateral common peroneal nerve (CPN) to assess the level of presynaptic inhibition (PSI_control). A brief conditioning vibratory stimulus was applied to the triceps surae tendon at the contralateral side (to activate preferentially Ia muscle afferents). The amplitude of the resulting H-reflex response (PSI_conditioned) was compared to the H-reflex under PSI_control, i.e., without the vibration. The interstimulus interval between the brief vibratory stimulus and the electrical shock to the CPN was −60 to 60 ms. The H-reflex conditioned by both stimuli did not differ from that conditioned exclusively by the ipsilateral CPN stimulation. In anesthetized cats, bilateral monosynaptic reflexes (MSRs) in the left and right L7 ventral roots were recorded simultaneously. Conditioning stimulation applied to the contralateral group I posterior biceps and semitendinosus (PBSt) afferents at different time intervals (0–120 ms) did not have an effect on the ipsilateral gastrocnemius/soleus (GS) MSR. An additional experimental paradigm in the cat using contralateral tendon vibration, similar to that conducted in humans, was also performed. No significant differences between GS-MSRs conditioned by ipsilateral PBSt stimulus alone and those conditioned by both ipsilateral PBSt stimulus and contralateral tendon vibration were detected. The present results strongly suggest an absence of effects from contralateral group I fibers on the presynaptic mechanism of MSR modulation in relaxed humans and anesthetized cats.

Active and inactive renin in primary and secondary adrenal insufficiency and during ACTH infusion

1983 ◽  
Vol 102 (2) ◽  
pp. 265-270
Author(s):  
Lutz Belkien ◽  
Petra Exner ◽  
Wolfgang Oelkers
Keyword(s):  
Plasma Renin ◽  
Renal Secretion ◽  
Young Males ◽  
Active Renin ◽  
Group I ◽  
Inactive Renin ◽  
Group Ii ◽  
Peripheral Activation ◽  
The Relationship ◽  
Stimulation Of

Abstract. Prolonged low-dose ACTH infusion leads to a transient stimulation of plasma renin activity (PRA) and angiotensin II. In part 1 of the present study (infusion of 10 IU of ACTH per day for 38 h into 6 normal young males), it was shown that the concentration of active renin (aPRC) increases in parallel to PRA. Thus, the rise in PRA is either due to net active renin secretion by the kidney or to increased conversion of inactive into active renin. Since the plasma concentration of inactive renin (iPRC) tended to rise rather than to fall during ACTH infusion, peripheral activation of inactive renin is probably not the cause of the rise in aPRC. Part 2 of the study consisted in the measurement of plasma ACTH, cortisol, PRA, aPRC and iPRC in 10 patients (group I) with primary adrenocortical insufficiency (8 Addisonians, 2 adrenalectomized Cushing's) and in 9 patients with hypopituitarism (group II) after short-term withdrawal of hydrocortisone substition therapy. ACTH was 1770 ± 390 pg/ml in the former and 20 ± 4 pg/ml in the latter group. PRA and aPRC were higher and the ratio iPRC:aPRC lower in group I than in group II. This might indicate stimulation of active renin formation by ACTH. However, it is unlikely that the higher aPRC levels in group I are due to increased peripheral activation of inactive renin, since the relationship between aPRC and the ratio iPRC:aPRC fell on the same curve in both groups. ACTH or an ACTH-dependent mechanism raises aPRC, probably by stimulating its renal secretion rather than by peripheral activation of inactive renin.

scholarly journals Development of an algorithm for the clinical and instrumental diagnosis of non-compression lumbar pain syndromes to optimize the use of puncture surgical techniques

2020 ◽  
pp. 27-34
Author(s):  
A. A. Kalinin ◽  
A. K. Okoneshnikova ◽  
Yu. Ya. Pestryakov ◽  
V. V. Shepelev ◽  
V. A. Byvaltsev
Keyword(s):  
Lumbar Spine ◽  
Surgical Techniques ◽  
Degenerative Changes ◽  
Lumbar Pain ◽  
Group Iii ◽  
Pain Syndromes ◽  
Group I ◽  
Neuroimaging Data ◽  
Group Ii ◽  
Stimulation Of

Background. Pain syndromes developing as a result of degeneration of the lumbar segments of the spine constitute a significant problem in modern vertebrology. The results of the application of preoperative diagnostic provocative tests are contradictory, and therefore the mixed effectiveness of puncture surgical techniques is recorded.Objective. To develop an algorithm for the clinical and instrumental diagnosis of non-compression lumbar pain syndromes to optimize the use of puncture surgical techniques.Material and Methods. The study included 923 patients who underwent provocative diagnostic techniques on the intervertebral disk (IVD) and the arched joints (AJ) between 2012 and 2017. Taking into account clinical and instrumental data, the following are made: in group I (n=246) – laser IVD nucleoplasty, in group II (n = 287) – laser denervation of FJ, in group III (n = 390) – simultaneous use of laser exposure to IVD and AJ. We analyzed the dynamics of the level of pain in the lumbar spine and lower extremities according to The Visual Analogue Scale and quality of life according to The Short Form-36 (SF-36) questionnaire.Results. When performing provocative diagnostic tests, the minimum number of adverse effects was recorded: 3.2% in group I, 2.4% in group II, and 2.1% in group III. After puncture methods of surgical treatment, a significant persistent decrease in the severity of preoperative pain was observed both in the lumbar spine (p = 0.002, p = 0.005 and p = 0.004, respectively) and in the lower extremities (p = 0.003, p = 0.001 and p = 0.005, respectively) A significant improvement in the physical and psychological components of health was also established in group I (p = 0.02 and p = 0.01, respectively), in group II (p = 0.01 and p = 0.03, respectively) and group III (p = 0.03 and p = 0.02, respectively).Conclusion. In the presence of neuroimaging parameters of IVD according to C. Pfirrmann III–IV and protrusion size 4–6 mm, minimal degenerative changes in the AJ according to A. Fujiwara I–II and D. Weishaupt I–II, as well as a positive disruption test, it is possible to perform laser nucleoplasty. In case of detection of neuroimaging data of IVD according to C. Pfirrmann I–II and protrusion size less than 4 mm, moderately pronounced degenerative changes in the AJ according to A. Fujiwara II–III and D. Weishaupt II–III, as well as positive paraarticular stimulation of AJ, laser denervation of AJ is recommended. When determining, according to neuroimaging data, moderate degenerative changes in IVD according to C. Pfirrmann over III and protrusion size 4–6 mm, as well as degeneration of AJ according to A. Fujiwara more than II and D. Weishaupt more than II, positive of samples and paraarticular stimulation of AJ, it is advisable to perform simultaneous surgical intervention in the volume of laser IVD nucleoplasty and laser denervation of AJ.

scholarly journals From the foot-mouth reflex to the hand-mouth reflex a continuum of responses to Appendicular Compression

1997 ◽  
Vol 55 (2) ◽  
pp. 186-192 ◽  
Author(s):  
Fleming S. Pedroso ◽  
Newra T. Rotta
Keyword(s):  
Mouth Opening ◽  
First Trimester ◽  
Lower Leg ◽  
Psychomotor Development ◽  
Response Patterns ◽  
Normal Children ◽  
Group I ◽  
Group Ii ◽  
Stimulation Of

We studied the mouth opening response to appendicular compression in two groups of children. This study was performed with the intention of testing the semiologic role of the act of mouth opening following stimulation of various regions, based on the hand mouth reflex of Babkin. Group I was formed by 33 normal children who underwent monthly follow up assessments since birth; and group II consisted of 50 children older than 6 months of age, known to have a neurologic deficit and a neuro-psychomotor development equivalent to that of a child in the first trimester of life. We observed that the normal mouth opening response in group I was more pronounced following compression of the hand and forearm when compared to compression of the arm (p<0.001). This response could persist for as long as the first 6 months of life. We were not able to elicit a mouth opening response following compression of the lower limb in this group. Among children from group II, we observed mouth opening responses to stimulation of all limb segments. Within the upper limb, the response was more pronounced following compression of the hand in comparison to the forearm (p<0.01), and forearm in comparison to the proximal arm (p<0.01). Stimulation of the foot was more effective in eliciting a mouth opening response when compared to equivalent stimulation of the lower leg (p<0.05). However, there was no statistical difference when responses to stimulation of the lower leg and thigh were compared. The presence of the previously unreported foot-mouth response may serve as an indicator of central nervous system compromise and could be associated with a poorer prognosis. We believe that our observations of the specific foot-mouth response patterns may serve as a marker of early neuro-psychomotor development dysfunction during childhood.

Sensory Integration in Presynaptic Inhibitory Pathways During Fictive Locomotion in the Cat

2002 ◽  
Vol 88 (1) ◽  
pp. 163-171 ◽  
Author(s):  
Ariane Ménard ◽  
Hugues Leblond ◽  
Jean-Pierre Gossard
Keyword(s):  
Presynaptic Inhibition ◽  
Muscle Afferents ◽  
Primary Afferent Depolarization ◽  
Skin Area ◽  
Fictive Locomotion ◽  
Step Cycle ◽  
Primary Afferent ◽  
Decerebrate Cat ◽  
Group I ◽  
Group I Afferents

The aim of this study is to understand how sensory inputs of different modalities are integrated into spinal cord pathways controlling presynaptic inhibition during locomotion. Primary afferent depolarization (PAD), an estimate of presynaptic inhibition, was recorded intra-axonally in group I afferents ( n = 31) from seven hindlimb muscles in L6–S1 segments during fictive locomotion in the decerebrate cat. PADs were evoked by stimulating alternatively low-threshold afferents from a flexor nerve, a cutaneous nerve and a combination of both. The fictive step cycle was divided in five bins and PADs were averaged in each bin and their amplitude compared. PADs evoked by muscle stimuli alone showed a significant phase-dependent modulation in 20/31 group I afferents. In 12/20 afferents, the cutaneous stimuli alone evoked a phase-dependent modulation of primary afferent hyperpolarization (PAH, n = 9) or of PADs ( n = 3). Combining the two sensory modalities showed that cutaneous volleys could significantly modify the amplitude of PADs evoked by muscle stimuli in at least one part (bin) of the step cycle in 17/31 (55%) of group I afferents. The most common effect (13/17) was a decrease in the PAD amplitude by 35% on average, whereas it was increased by 17% on average in the others (4/17). Moreover, in 8/13 afferents, the PAD reduction was obtained in 4/5 bins i.e., for most of the duration of the step cycle. These effects were seen in group I afferents from all seven muscles. On the other hand, we found that different cutaneous nerves had quite different efficacy; the superficial peroneal (SP) being the most efficient (85% of trials) followed by Saphenous (60%) and caudal sural (44%) nerves. The results indicate that cutaneous interneurons may act, in part, by modulating the transmission in PAD pathways activated by group I muscle afferents. We conclude that cutaneous input, especially from the skin area on the dorsum of the paw (SP), could subtract presynaptic inhibition in some group I afferents during perturbations of stepping (e.g., hitting an obstacle) and could thus adjust the influence of proprioceptive feedback onto motoneuronal excitability.

scholarly journals The influence of acute and chronic hypercalcemia on the parathyroid hormone response to hypocalcemia in rabbits

2002 ◽  
pp. 411-418 ◽  
Author(s):  
S Bas ◽  
E Aguilera-Tejero ◽  
JC Estepa ◽  
B Garfia ◽  
I Lopez ◽  
...  
Keyword(s):  
Parathyroid Hormone ◽  
Elevated Level ◽  
Renal Mass ◽  
Hormone Response ◽  
Group Iii ◽  
Surgical Reduction ◽  
Group I ◽  
Group Ii ◽  
Acute Hypercalcemia ◽  
Stimulation Of

OBJECTIVE: To investigate the influence of acute and chronic hypercalcemia on the parathyroid hormone (PTH) response to hypocalcemia. DESIGN: The PTH response to hypocalcemia has been evaluated in three groups of rabbits: Group I, normal rabbits, Group II, normal rabbits subjected to an acute hypercalcemic clamp (induced by CaCl(2) infusion) and Group III, rabbits with chronic hypercalcemia (due to surgical reduction of renal mass). RESULTS: In Group I (baseline Ca(2+)=1.69+/-0.02 mM), hypocalcemia resulted in stimulation of PTH secretion which reached a maximum (PTHmax) of 91.7+/-6.4 pg/ml. In rabbits from Group II, which also had normal baseline Ca(2+) (1.70+/-0.02 mM), plasma Ca(2+) was maintained at an elevated level for 2 h, at around 2.05 mM. The PTH response to hypocalcemia in Group II was attenuated and the PTHmax in these rabbits was 45.6+/-7.4 pg/ml. In rabbits from Group III, baseline Ca(2+) was elevated (2.06+/-0.06 mM) for 1 month. The PTH response to hypocalcemia in Group III was esentially the same as in Group I and PTHmax reached levels of 94.8+/-9.9 pg/ml. CONCLUSIONS: A difference in PTH response to hypocalcemia has been found in rabbits after exposure to either acute or chronic hypercalcemia. After acute hypercalcemia, an attenuated PTH response to hypocalcemia has been identified. Chronic hypercalcemia, however, did not influence the PTH response to hypocalcemia.

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