In vivo and in vitro vasoactive reactions of coronary arteriolar microvessels to nitroglycerin

The actions of nitroglycerin on the coronary microcirculation are controversial, with some laboratories reporting that coronary arterioles dilate to the drug and others reporting that they do not. Our goal was to reconcile these disparate observations. Specifically, we hypothesized that dilation of coronary arterioles by nitroglycerin is overwhelmed by intrinsic autoregulatory escape mechanisms. Accordingly, we projected that coronary arterioles would show transient, but not sustained, dilation to nitroglycerin in vivo. Furthermore, we hypothesized that isolated coronary arterioles would show sustained dilation to the drug, because intrinsic escape mechanisms would be absent under these conditions. To test these hypotheses, we measured diameter changes of canine coronary microvessels in vivo during continuous nitroglycerin administration (intracoronary infusion or epicardial suffusion) using intravital fluorescent microscopy (n = 17 dogs) at two time points: early (1-3 min), when coronary artery blood flow velocity was increased, and late (15-20 min), after blood flow velocity returned to control. Tb study responses of coronary arterioles in the absence of autoregulatory influences, we measured the diameter of isolated canine coronary arterioles to varying doses of nitroglycerin (n = 8 vessels, maximal diameter 81 +/- 4 microns). During the early phase of nitroglycerin infusion (1,3, and 10 micrograms.kg-1.min-1), coronary arterioles dilated by 4 +/- 1, 7 +/- 2, and 13 +/- 2% (all P < 0.05), whereas small arteries dilated by 1 +/- 2, 3 +/- 1, and 4 +/- 1%, respectively (P < 0.05 for the higher doses). Coronary artery blood velocity measured increased by 45 +/- 15% (3 micrograms.kg-1.min-1, P < 0.05). Suffusion of nitroglycerin (10(-5) M) dilated coronary arterioles, but not small arteries, by 17 +/- 5% (P < 0.05) between 1 and 3 min. After 15-20 min of nitroglycerin (3 micrograms.kg-1.min-1 by intracoronary infusion), diameters of coronary arterioles and coronary artery blood velocity returned to control, whereas dilation of small arteries remained significant at 4 +/- 1%. Coronary arteriolar dilation by epicardial suffusion of nitroglycerin also waned to control values by 15-20 min, whereas dilation of small arteries was observed: 5 +/- 2% (P < 0.05). In vitro, nitroglycerin caused dose-dependent dilation of coronary arterioles to their maximal diameter, which was sustained for 20 min. Thus nitroglycerin dilates coronary arterioles and small arteries. The dilation in vivo is transient for arterioles but sustained for arteries. In vitro, the dilation is sustained. Because microvessels in vitro are capable of sustaining dilation for 20 min, we conclude that the waning of arteriolar dilation in vivo is related to autoregulatory escape from dilation by nitroglycerin.

Expression of vascular escape: conductance or resistance?

Vascular escape is that phenomenon whereby a tachyphylaxis occurs in the vasoconstriction of an arteriole to a constant sympathetic stimulation. Vascular escape, in vivo, is primarily a blood flow event. Calculated resistance, as an index of vascular tone, does not consistently describe the responses of the arterioles undergoing vascular escape. Conductance, which is the inverse of resistance, obviates several of the errors produced by the use of resistance. In this study, we illustrate this issue using hypothetical and experimental data. Escape responses were calculated in terms of resistance and conductance and plotted against blood flow escape responses. Resistance escape responses were nonlinearly related to blood flow escapes and overestimated vascular escape with both hypothetical and experimental data. Conductance escape responses were linearly related to flow escape responses and consistently described vascular escape. We therefore conclude that conductance is a better index of vascular tone to express vascular escape. arterial vascular tone; vasoconstriction; autoregulatory escape; blood flow; blood pressure Submitted on July 18, 1991 Accepted on November 11, 1991

Modulation of autoregulatory escape by capsaicin-sensitive afferent nerves in rat stomach

Visceral C fibers are stimulated by ischemia and hypoxia, which can be produced by intense vasoconstriction. Epinephrine applied to the gastric submucosa produces a marked vasoconstriction followed by autoregulatory escape. We hypothesize that the autoregulatory escape from epinephrine-induced vasoconstriction in the rat stomach is mediated partly by capsaicin-sensitive C fibers. Functional ablation of these afferent fibers by high-dose systemic capsaicin pretreatment will significantly reduce the magnitude of the autoregulatory escape. Rats received capsaicin (125 mg/kg sc) 10 days before blood flow studies to produce functional impairment of the capsaicin-sensitive afferent nerves. Control rats received vehicle. Under urethan anesthesia, a small area (2 mm diam) of the serosa from the anterior gastric wall was removed to expose the submucosa. The tip of a side-viewing laser-Doppler flow probe was placed inside the stomach directly beneath the exposed submucosa. At 20-min intervals, 20 microliters of buffer, 5 x 10(-4) M epinephrine, 1.6 x 10(-4) M capsaicin, or 3.3 x 10(-2) M histamine was applied topically to the exposed submucosa, with saline washes between applications at 10 min after each application. Blood pressure and laser-Doppler flow signals were monitored continuously. The escape index during the period of epinephrine application was significantly lower in the capsaicin-pretreated rats (0.239 +/- 0.046) than in the vehicle-pretreated rats (0.474 +/- 0.079). Functional ablation of the capsaicin-sensitive afferent fibers was confirmed by a significant blockade of the vasodilatation induced by topical capsaicin. Histamine-induced vasodilatation was unaffected.(ABSTRACT TRUNCATED AT 250 WORDS)

Role of H+ and alpha 2-receptors in escape from sympathetic vasoconstriction

In a previous study, we noted that mesenteric venous pH falls during the reductions in intestinal blood flow caused by sympathetic stimulation and that alpha 2-receptor antagonists enhanced autoregulatory escape (the partial recovery that blood flow undergoes despite sustained sympathetic stimulation). In addition, other studies indicated that increased [H+] selectively inhibits the responsiveness of postjunctional alpha 2-receptors to norepinephrine (NE). Therefore, we investigated the role of H+ in escape by 1) measuring the rate of unbuffered H+ release during sympathetic stimulation in isolated loops of canine small bowel, 2) infusing acidic buffer intra-arterially and determining the effects of acidosis on sympathetic vasoconstriction and escape, 3) ascertaining the effects of acidosis on the release rate of endogenous NE during sympathetic stimulation, and 4) determining whether acidosis exerts effects in vivo on post-junctional responses to the selective alpha 1- and alpha 2-agonists, phenylephrine and clonidine, respectively. Our findings were that 1) the rate at which the gut released H+ into blood increased during sympathetic stimulation, 2) infusing acidic buffer to lower venous pH from 7.3 to 7.1 attenuated the initial vasoconstrictor response after 30 s of stimulation, 3) acidosis caused blood flow to return further toward control despite continued stimulation and thus enhanced escape, 4) acidosis did not impair NE release at either 30 s or 6 min of stimulation, and 5) acidosis inhibited the intestinal vasoconstrictor effects of selective alpha 2- but not alpha 1-agonists. The results support the hypothesis that escape from sympathetic vasoconstriction occurs, in part, because increased [H+] inhibits alpha 2-mediated postjunctional responses to neuronally released NE.

Norepinephrine release during autoregulatory escape: effects of alpha 2-receptor blockade

The partial recovery that intestinal blood flow undergoes during continued sympathetic nerve stimulation is termed autoregulatory escape. This study tested two hypotheses that might explain escape: 1) diminishing norepinephrine (NE) release during sustained stimulation and 2) an alpha 2-receptor-mediated competition between local and neural control mechanisms. The rates of NE release before and during stimulation of the perivascular sympathetic nerves were determined by measuring blood flow in isolated loops of canine small intestine and assaying the concentrations of NE in arterial and venous blood. The presence of functional alpha 2-receptors was demonstrated by clonidine injections, and the effects of alpha 2-receptor blockade were studied during yohimbine infusions. The time course of NE release was inconsistent with a cause-effect relationship; NE release was greatest during the phase when resistance had already escaped. Deliberately altering NE release by changing the stimulus duration did not affect escape. The study demonstrated 1) that diminished NE release during continued sympathetic stimulation does not occur and cannot account for escape, 2) that resistance vessels in the canine intestinal circulation possess functional alpha 2-receptors which are responsible for part of the vasoconstriction caused by sympathetic stimulation, 3) that blockade of presynaptic alpha 2-receptors significantly enhanced NE release during the initial 30-s period but not during the escape phase, and 4) that alpha 2-receptor blockade enhances autoregulatory escape. Altogether these findings indicate that the postsynaptic alpha 2-receptors on intestinal resistance vessels deserve further investigation as the possible site at which local and neural mechanisms compete to influence vascular resistance.

Sensory nerves mediate neurogenic escape in rat gut

We investigated the involvement of primary sensory nerves in intestinal autoregulatory escape induced by postganglionic nerve stimulation (NS) in anesthetized rats. Anterior mesenteric artery (AMA) blood flow velocity (BF) was measured with a pulsed Doppler flowmeter. Periarterial NS elicited an abrupt fall in BF, which was followed by a recovery in BF toward the basal value, despite sustained NS. This recovery from NS constituted the neurogenic escape phenomenon. Vasoconstrictor responses to NS were abolished by periarterial application of tetrodotoxin. Acute, surgical interruption of proximal periarterial nerves had no effect on BF responses to distal NS, suggesting a peripheral rather than a central nervous mechanism for the escape phenomenon. Escape from NS-induced vasoconstriction was significantly inhibited by prior administration of the selective sensory neurotoxin capsaicin as either subcutaneous injection in neonatal life, acute application to periarterial nerves, or acute injection into the jejunal lumen. In rats pretreated 24 h with reserpine, NS provoked a vasodilator response that was inhibited by intrajejunal capsaicin. Increases in arterial blood pressure (BP) and heart rate observed during NS were blocked by periarterial (but not intrajejunal) application of capsaicin. Transmural electrical field stimulation elicited significantly greater nerve-induced contractions in AMA rings from control rats. Our findings support the hypothesis that postganglionic NS activates both vasoconstrictor sympathetic nerve branches and vasodilator afferent C-fibers. The latter nerves release vasodilator peptides in the periphery during continuous low frequency NS that appear to be essential for autoregulatory escaped in our model.

Gastric oxygen uptake during autoregulatory escape from sympathetic stimulation

To assess the effects of sympathetic stimulation on gastric blood flow and oxygen utilization, the perivascular nerves were stimulated at 2, 4, 6, and 8 Hz in chambered segments of canine gastric corpus perfused at constant pressure. Spectrophotometric arteriovenous oxygen difference and electromagnetic blood flow were recorded continuously. Except at the lowest frequency of stimulation (2 Hz), total blood flow exhibited autoregulatory escape, i.e., blood flow decreased initially but then returned toward control. The fall in total blood flow at the onset of sympathetic stimulation was smaller at 2 Hz than at 4 Hz, but stimulation at 6 and 8 Hz caused no further reductions in total blood flow. However, at all frequencies, total blood flow escaped to the same steady-state value (approximately 17 ml.min-1.100 g-1). Although total blood flow was still less than control (approximately 25 ml.min-1.100 g-1), oxygen extraction increased proportionately so that oxygen consumption was not significantly less than control at any frequency of stimulation. We conclude that autoregulatory escape from sympathetic stimulation is mediated by local mechanisms acting to maintain tissue oxygenation in the stomach.

Metabolic and circulatory responses of normoxic skeletal muscle to whole-body hypoxia

Whole-body hypoxia may increase peripheral O2 demand because it increases catecholamine calorigenesis, an effect attributable to beta 2-adrenoceptors. We tested these possibilities by pump-perfusing innervated hindlimbs in eight dogs with autologous blood kept normoxic by a membrane oxygenator while ventilating the animals for 40 min with 9% O2 in N2 (NOB group). Similar periods of normoxic ventilation preceded and followed the hypoxic period. A second group (n = 8, beta B) was pretreated with the specific beta 2 blocker ICI 118,551. Hindlimb O2 uptake was elevated by 25 min of hypoxia in NOB, whereas whole-body O2 uptake was reduced. Limb O2 uptake remained elevated in recovery, but all effects on limb O2 uptake were absent in beta B. Hindlimb resistance and perfusion pressure increased in hypoxia in both groups, and there was little evidence of local escape from reflex vasoconstriction. These results clearly indicated that global hypoxia increased O2 demand in muscle when the local O2 supply was not limited and that beta 2-receptors were necessary for this response. Autoregulatory escape of limb muscle blood flow from centrally mediated vasoconstriction during whole-body hypoxia was also shown to be practically nil, if normoxia was maintained in the limb.

Autoregulatory escape from norepinephrine infusion: roles of adenosine and histamine

Stimulation of sympathetic fibers or infusion of norepinephrine (NE) into the superior mesenteric artery (SMA) leads to an initial decrease in intestinal blood flow, which is followed by a return of flow toward the base-line value (autoregulatory escape) despite continued nerve stimulation or NE infusion. Although the mechanisms responsible for “autoregulatory escape” have not been defined, accumulation of vasodilator metabolites is frequently invoked to explain this phenomenon. Inasmuch as histamine and adenosine exist in high concentrations in the intestinal mucosa and both are potent vasodilators, we examined the effects of chlorpheniramine (an H1 blocker) and adenosine deaminase (degrades adenosine) on autoregulatory escape from NE infusion. In autoperfused piglet intestinal preparations, we measured SMA blood flow and the arteriovenous oxygen difference during intra-arterial NE infusion before and after blockade with chlorpheniramine or adenosine deaminase. Adenosine deaminase pretreatment increased the peak vasoconstrictor and reduced the steady-state escape responses to NE infusion. Chlorpheniramine did not affect either the vasoconstrictor or escape responses. The oxygen uptake changes induced by NE infusion were not dramatically modified by either treatment. These results indicate that adenosine but not histamine is responsible for at least part of the escape of intestinal blood flow from NE infusion.