Renovascular BKCa channels are not activated in vivo under resting conditions and during agonist stimulation

We investigated the role of large-conductance Ca2+-activated K+ (BKCa) channels for the basal renal vascular tone in vivo. Furthermore, the possible buffering by BKCa of the vasoconstriction elicited by angiotensin II (ANG II) or norepinephrine (NE) was investigated. The possible activation of renal vascular BKCa channels by cAMP was investigated by infusing forskolin. Renal blood flow (RBF) was measured in vivo using electromagnetic flowmetry or ultrasonic Doppler. Renal preinfusion of tetraethylammonium (TEA; 3.0 μmol/min) caused a small reduction of baseline RBF, but iberiotoxin (IBT; 0.3 nmol/min) did not have any effect. Renal injection of ANG II (1–4 ng) or NE (10–40 ng) produced a transient decrease in RBF. These responses were not affected by preinfusion of TEA or IBT. Renal infusion of the BKCa opener NS-1619 (90.0 nmol/min) did not affect basal RBF or the response to NE, but it attenuated the response to ANG II. Coadministration of NS-1619 with TEA or IBT abolished this effect. Forskolin caused renal vasodilation that was not inhibited by IBT. The presence of BKCa channels in the preglomerular vessels was confirmed by immunohistochemistry. Despite their presence, there is no indication for a major role for BKCa channels in the control of basal renal tone in vivo. Furthermore, BKCa channels do not have a buffering effect on the rat renal vascular responses to ANG II and NE. The fact that NS-1619 attenuates the ANG II response indicates that the renal vascular BKCa channels can be activated under certain conditions.

NO-independent mechanism mediates tempol-induced renal vasodilation in SHR

We investigated whether tempol, a superoxide dismutase mimetic, affected renal hemodynamics and arterial pressure in spontaneously hypertensive rats (SHR) and Sprague-Dawley (SD) rats. We also examined whether tempol affected exaggerated renal vasoconstrictor responses to ANG II in SHR. To test whether the effects of tempol were due to a restored NO system, we used the NOS inhibitor Nw-nitro-l-arginine methyl ester (l-NAME). Renal blood flow (RBF) and mean arterial pressure (MAP) were measured in vivo by electromagnetic flowmetry and arterial catheterization in 10- to 12-wk-old anesthetized SHR and SD rats. Systolic arterial pressure (SAP) was measured in conscious rats using the tail cuff method. Tempol (1 mM) was given in the drinking water to SD (SD-T) and SHR (SHR-T) for 5–7 days for RBF measurements and for 15 days for SAP measurements. Age-matched SD (SD-C) and SHR (SHR-C) were used as controls. ANG II (1–4 ng) was administered as a bolus via a renal artery catheter. l-NAME was administered intravenously for 15–20 min. Renal vascular resistance (RVR) was elevated in SHR-C compared with SD-C. In SHR-T, baseline RVR was not different from SD-C and SD-T rats. Tempol had no effect on RVR in SD. l-NAME elevated RVR to the same extent in all four groups. Arterial pressure was not affected by tempol. The RVR responses to ANG II were higher in SHR-C than in the SD-C group. ANG II responses were not different between SHR-T and SD-T. Overall, tempol reduced the renovascular responses to ANG II in SHR. l-NAME elevated the effects of ANG II in SD-C rats but had no effect on the ANG II responses in the other groups. Thus l-NAME treatment did not influence tempol’s effects on baseline RVR or ANG II responses. We conclude that in SHR, tempol has a significant renal vasodilator effect and that it normalizes the increased renovascular ANG II sensitivity. As the effects of l-NAME are not greater in SHR-T rats, it is not likely that the elevated renal resistance and ANG II sensitivity in SHR are due to reactive oxygen species-induced quenching of nitric oxide.

Effects of chloride channel blockers on rat renal vascular responses to angiotensin II and norepinephrine

The aim of the present study was to investigate the role of Ca2+-activated Cl- channels in the renal vasoconstriction elicited by angiotensin II (ANG II) and norepinephrine (NE). Renal blood flow (RBF) was measured in vivo using electromagnetic flowmetry. Ratiometric photometry of fura 2 fluorescence was used to estimate intracellular free Ca2+ concentration ([Ca2+]i) in isolated preglomerular vessels from rat kidneys. Renal arterial injection of ANG II (2-4 ng) and NE (20-40 ng) produced a transient decrease in RBF. Administration of ANG II (10-7 M) and NE (5 × 10-6 M) to the isolated preglomerular vessels caused a prompt increase in [Ca2+]i. Renal preinfusion of DIDS (0.6 and 1.25 μmol/min) attenuated the ANG II-induced vasoconstriction to ∼35% of the control response, whereas the effects of NE were unaltered. Niflumic acid (0.14 and 0.28 μmol/min) and 2-[(2-cyclopentenyl-6,7-dichloro-2,3-dihydro-2-methyl-1-oxo-1 H-inden-5-yl)oxy]acetic acid (IAA-94; 0.045 and 0.09 μmol/min) did not affect the vasoconstrictive responses of these compounds. Pretreatment with niflumic acid (50 μM) or IAA-94 (30 μM) for 2 min decreased baseline [Ca2+]i but did not change the magnitude of the [Ca2+]i response to ANG II and NE in the isolated vessels. The present results do not support the hypothesis that Ca2+-activated Cl- channels play a crucial role in the hemodynamic effects of ANG II and NE in rat renal vasculature.

Nitric oxide mediates sympathetic vasoconstriction at supraspinal, spinal, and synaptic levels

The purposes of this study were to investigate the level of the sympathetic nervous system in which nitric oxide (NO) mediates regional sympathetic vasoconstriction and to determine whether neural mechanisms are involved in vasoconstriction after NO inhibition. Ganglionic blockade (hexamethonium), α1-receptor blockade (prazosin), and spinal section at T1 were used to study sympathetic involvement. NO was blocked with N ω-nitro-l-arginine methyl ester (l-NAME). Regional blood flow in the mesenteric and renal arteries and terminal aorta was monitored by electromagnetic flowmetry in conscious rats.l-NAME (3–5 mg/kg iv) increased arterial pressure and peripheral resistance. Ganglionic blockade (25 mg/kg iv) significantly reduced the increase in resistance in the mesentery and kidney in intact and spinal-sectioned rats. Ganglionic blockade significantly decreased hindquarter resistance in intact rats but not in spinal-sectioned rats. Prazosin (200 μg/kg iv) significantly reduced the increased hindquarter resistance. We concluded that NO suppresses sympathetic vasoconstriction in the mesentery and kidney at the spinal level, whereas hindquarter tone is mediated at supraspinal and synaptic levels.

ESPVR of in situ rat left ventricle shows contractility-dependent curvilinearity

We developed a miniaturized conductance catheter for in situ rat left ventricular (LV) volumetry. After the validation study of the conductance volumetry in 11 rats, we characterized the end-systolic pressure-volume relationship (ESPVR) in 24 sinoaortic-denervated, vagotomized and urethan-anesthetized rats. Stroke volume (SV) measured with the conductance catheter correlated closely with that measured by electromagnetic flowmetry ( r > 0.95). No significant difference was found between the in situ LV end-diastolic volumes measured by conductance volumetry and postmortem morphometry; a linear regression analysis indicated that the correlation coefficient was 0.934, that the slope was not significantly different from 1, and that the intercept was not significantly different from 0. During cardiac sympathotonic conditions, the ESPVR was curvilinear. The estimated slope of ESPVR (end-systolic elastance, E es) by quadratic curve fitting at end-systolic pressure of 100 mmHg was 2,647 ± 846 mmHg/ml. Bilateral cervical and stellate ganglionectomy depressed contractility and made the ESPVR linear; a quadratic equation did not improve the fit. E es was 946 ± 55 mmHg/ml with the volume-axis ( V 0) intercept of 0.076 ± 0.007 ml. Administration of propranolol (1 mg/kg) further reduced E es (573 ± 61 mmHg/ml, P < 0.001) and increased V 0 slightly (0.091 ± 0.011 ml). We conclude that the conductance catheter method is useful for the assessment of the ESPVR of the in situ rat left ventricle and that the ESPVR displays contractility-dependent curvilinearity.

Interactions of cAMP-mediated vasodilators with angiotensin II in rat kidney during hypertension

In previous studies [C. Chatziantoniou and W.J. Arendshorst. Am. J. Physiol. 263 (Renal Fluid Electrolyte Physiol. 32): F573-F580, 1992], we reported that vasodilator prostaglandins (PGs) are defective in buffering the angiotensin II (ANG II)-induced vasoconstriction in the renal vasculature of spontaneously hypertensive rats (SHR). The purpose of the present study was to determine whether this defect in SHR kidneys is specific to PGs or generalized to the action of vasodilators and to gain insight into which intracellular signal(s) mediates this abnormality. Renal blood flow (RBF; electromagnetic flowmetry) was measured in 7 wk-old anesthetized, euvolemic SHR and normotensive Wistar-Kyoto (WKY) rats pretreated with indomethacin to avoid interactions with endogenous PGs. ANG II (2 ng) was injected into the renal artery before and during continuous intrarenal infusion of fenoldopam [DA1 receptor agonist and G protein-dependent stimulator of adenosine 3',5'-cyclic monophosphate (cAMP)], forskolin (G protein-independent stimulator of cAMP), dibutyryl-cAMP (soluble cAMP), and acetylcholine (cGMP stimulator). Each vasodilator was infused at a low dose that did not affect baseline arterial pressure or RBF. In the control period, ANG II reduced RBF by 50% in both strains. Infusion of fenoldopam significantly blunted the ANG II-induced vasoconstriction in WKY, but not in SHR. In contrast, forskolin, dibutyryl-cAMP, and acetylcholine effectively buffered the vasoconstriction due to ANG II in both SHR and WKY. These results suggest that renal vasodilators acting through receptor binding to stimulate the cAMP signaling pathway are ineffective in counteracting the ANG II-induced vasoconstriction in SHR kidneys. (ABSTRACT TRUNCATED AT 250 WORDS)

Validity of thermal dilution technique for measurement of cardiac output in rats

The validity of the thermal dilution technique for the measurement of cardiac output was verified in experiments on a circulation model and on anesthetized rats under open- and closed-chest conditions. In the circulation model thermal dilution was compared with direct (Fdir) and electromagnetic (F(elm)) flowmetry. Flow values measured in the circulation model with the thermal dilution (Fth) technique correspond well with direct flowmetry (Fth = 0.92 Fdir + 7.0; r = 0.888) and with electromagnetic flowmetry (Fth = 0.95 F(elm) + 1.2; r = 0.990). In the anesthetized rat cardiac output was determined with thermal dilution and simultaneously with Fick's method and/or with electromagnetic flowmetry. Fick's method and electromagnetic flowmetry resulted in identical cardiac output values (COFick = 0.95 COelm; r = 0.865), whereas the thermal dilution technique yielded unequivocally higher values. The extent of overestimation is much more pronounced at low cardiac output than at a high output. The study clearly demonstrates that this overestimation is due to heat diffusion, which is obviously of greater significance in small animals than in large animals or humans. Therefore, the thermal dilution technique is not appropriate for the measurement of cardiac output in the rat.