Mechanism of the decrease in coronary blood flow after beta-blockade in conscious dogs

Heterogeneous microvascular coronary alpha-adrenergic vasoconstriction

We tested the hypothesis that humoral or neurogenic alpha-adrenergic activation in the coronary circulation would produce heterogeneous vascular reactions. To accomplish this, the epicardial coronary microcirculation was viewed through an intravital microscope using stroboscopic epi-illumination. Microvascular diameters were measured under control conditions during beta-adrenergic blockade (propranolol 1 mg/kg) and beta-adrenergic blockade with pacing; during coronary alpha-adrenergic activation in the presence of beta-adrenergic blockade with three doses of norepinephrine infusion (0.1, 0.5, and 1.0-2.0 micrograms/kg/min) or three frequencies of bilateral stellate nerve stimulation (2, 10, and 20 Hz); and during combined alpha- and beta-adrenergic blockade (phentolamine 2 mg/kg and propranolol 1 mg/kg). Diameters of both arterial and venous vessels were reduced during beta-adrenergic blockade but returned back to baseline with pacing. At the lowest level of norepinephrine infusion or frequency of bilateral stellate stimulation, microvessel constriction was not observed. At the higher doses of norepinephrine a -5.1 +/- 0.9% (1.0-2.0 micrograms/kg/min) and a -4.0 +/- 1.1% (0.5 micrograms/kg/min) decrease in diameter of arterial vessels greater than 100 microns in diameter were observed (p less than 0.05). At 10 Hz and 20 Hz of stellate stimulation, diameter decreased by -4.8 +/- 1.9% and -4.4 +/- 2.1%, respectively, in these relatively large vessels. Small coronary arterioles (less than 100 microns diameter) dilated significantly during the highest levels of nerve stimulation (9.2 +/- 2.5% increase in diameter) or infusion rate of norepinephrine (13.6 +/- 2.7% increase in diameter) (p less than 0.05). These constrictor and dilator responses were abolished following combined alpha- and beta-adrenergic blockade. Norepinephrine infusion resulted in a decrease in diameter of coronary veins and venules (7.2 +/- 1.3%) (p less than 0.05), whereas stellate stimulation did not significantly reduce venous and venular diameters. In summary, the coronary venous and venular vasculature responds to alpha-adrenergic activation from circulating norepinephrine but is not affected by stellate stimulation. In contrast, stellate stimulation and norepinephrine infusion elicit similar responses in the coronary arterial and arteriolar microvasculature. Constriction occurs in vessels greater than 100 microns in diameter, whereas dilation predominates in vessels less than 100 microns in diameter. Such heterogeneous arterial responses would undoubtedly result in a redistribution of coronary vascular resistance toward larger coronary arteries and arterioles.

Transmural differences in sympathetic coronary constriction during exercise in the presence of coronary stenosis

The goal of this study was to determine the effect of sympathetic neural activation on the transmural distribution of myocardial perfusion distal to a flow-limiting coronary artery stenosis. Treadmill exercise in conscious dogs was used as a physiological stimulus to activate the sympathetic nervous system. In the experimental model, the anterior region of the circumflex artery was innervated, but the posterior circumflex region was treated with phenol to produce regional sympathectomy within the stenotic territory. Myocardial perfusion to innervated and sympathectomized left ventricular regions was measured before and after inflation of the occluder to reduce distal coronary pressure to 45 mm Hg. Measurements were obtained during control conditions with the animal standing on the treadmill, during inflation of the occluder with the animal standing, during exercise alone, during exercise with beta-adrenergic blockade, and during exercise with combined alpha- and beta-adrenergic blockade. Exercise (6 km/hr) resulted in a marked increase in heart rate from 128 +/- 9 (standing) to 218 +/- 7 beats/min. beta-Adrenergic blockade blunted the tachycardia during exercise (146 +/- 6 beats/min). Under control conditions (while standing), there were no differences in myocardial perfusion between the innervated and sympathectomized regions, 187 +/- 26 and 181 +/- 24 ml.min-1.100 g-1, respectively. During exercise or in combination with beta-adrenergic blockade, subepicardial perfusion was significantly less (18-25%) in the innervated stenotic region than that in the sympathectomized stenotic region. In contrast, subendocardial perfusion was significantly greater in the innervated stenotic region (17-26%) than that in the sympathectomized stenotic region. The subendocardial-to-subepicardial blood flow ratio during exercise was 0.60 +/- 0.08 in the innervated stenotic region and 0.42 +/- 0.07 in the sympathectomized stenotic region (p less than 0.05). During exercise with beta-adrenergic blockade, the endocardial-to-epicardial blood flow ratios in the innervated and sympathectomized stenotic regions were 0.47 +/- 0.09 and 0.37 +/- 0.07, respectively (p less than 0.05). These differences were abolished during alpha- and beta-adrenergic blockade. These data indicate that alpha-adrenergic coronary constriction distal to a flow-limiting stenosis facilitates redistribution of blood flow toward the subendocardium. This redistribution was produced by alpha-adrenergic constriction in the outer layers of the left ventricle.

Contractions of amphibian Wolffian duct in response to acetylcholine, norepinephrine, and arginine vasotocin

The regulation of sperm transport through the Wolffian duct of male amphibians is poorly understood. These experiments were conducted using rough-skinned newts (Taricha granulosa) to determine if Wolffian ducts are capable of contracting in vitro and, if so, to characterize the contractile responses to acetylcholine (ACh), norepinephrine (NE), and neurohypophysial hormones. Dose-response curves for NE and ACh, which were prepared by measuring isometric contractions, are similar to those reported for mammalian vas deferens. For NE, the minimum effective dose and ED50 were found to be 1 X 10(-5)M and 4.17 X 10(-5)M, respectively. For ACh, the minimum effective dose was 3.2 X 10(-8)M and the ED50 was 1.37 X 10(-5)M. Alpha-adrenoreceptors appear to mediate the contractile responses to NE because phentolamine (10(-5)M) blocked or attenuated the response to NE (10(-6)M, 10(-5)M or 10(-4) M). Beta-adrenoreceptors appear to mediate relaxation because dichloroisoproterenol (10(-5)M) enhanced the response to 10(-5)M NE. The contractile response to three neurohypophysial hormones were also investigated. Arginine vasotocin was more effective in eliciting contractions than oxytocin. The effect of lysine vasopressin was intermediate between arginine vasotocin and oxytocin. These experiments demonstrate that amphibian (Taricha) Wolffian ducts contract in vitro in response to neurotransmitters and neurohypophysial hormones. The contractile response to neurotransmitters occurs in a dose-dependent manner; the response to neurohypophysial hormones is hormone specific.

Residual effects when chronic propranolol therapy is discontinued within 48 hours of cardiopulmonary bypass

Recommendations regarding the safe waiting period between discontinuing chronic oral propranolol therapy and beginning cardiopulmonary bypass have varied from a few hours to 2 weeks. In the present study, utilizing adult dogs, propranolol was discontinued 8 or 48 hours prior to surgery. A reduction in cardiac output and elevations of left ventricular end-diastolic pressure, peak systolic pressure, and systemic resistance were noted when cardiac function was evaluated following the induction of anesthesia and prior to undertaking cardiopulmonary bypass. The magnitude of these differences was directly related to the degree of volume loading and inversely related to the interval between the last dose of propranol and the determination of cardiac function. Reduction of heart rate was the most evanescent of propranolol's hemodynamic effects as the marked bradycardia which persisted throughout the course of propranolol therapy was no longer evident 8 hours after the last oral dose of the drug. Following total cardiopulmonary bypass of 1 hour's duration, undertaken 8 hours after the last oral dose of propranolol, cardiac output and left ventricular end-diastolic pressure had returned to normal but peak systolic pressure and systemic resistance remained significantly elevated, When 48 hours had elapsed between discontinuing propranolol and beginning cardiopulmonary bypass, postbypass cardiac function was essentially normal with only slight persistent elevations of peak systolic pressure and systemic resistance detected. When the combined effects of ischemic heart disease and propranolol therapy, the altered metabolic and hemodynamic effects of different routes of drug administration, and the varying durations of cardiopulmonary bypass are taken into consideration, some of the discrepancies between previously reported clinical and experimental findings regarding the duration of persistent propranolol effects can be understood. The clinical course is usually benign in patients who have received propranolol to within a few hours of surgery without specific indication. However, it is often complicated when the drug is continued until just prior to surgery in patients dependant on propranolol for pain or arrhythmia control. In patients demonstrating propranolol dependence, control of symptoms with intra-aortic balloon counterpulsation is recommended followed by the gradual withdrawal of propranolol and elective aortocoronary bypass surgery.