Coronary vasomotor disorders during hypoxia-reoxygenation: do calcium channel blockers play a protective role?

Selective glycolysis blockade in guinea pig pulmonary artery and aorta reverses contractile and electrical responses to acute hypoxia

The goal of this study was to clarify the mechanisms of hypoxic pulmonary vasoconstriction (HPV) reversal following selective glycolysis blockade and to assess possible contribution of endothelial electrogenesis to this phenomenon as a trigger mechanism. We compared smooth muscle (SM) contractility and endothelial cell (EC) membrane potential (MP) during acute hypoxia before and after glycolysis blockade. MPs were recorded from the endothelium of guinea pig pulmonary artery (GPPA) and thoracic aorta (GPTA) using the patch-clamp technique. Acute hypoxia caused hyperpolarization in GPTA EC, while EC from GPPA were depolarized. Also, acute hypoxia elicited constriction in isolated GPPA and dilatation in GPTA. Selective glycolysis inhibition always reversed both electrical and contractile responses in GPPA to hypoxia, but in GPTA this only occurred in 30% of experiments. It is likely that an unknown glycolysis-driven mechanism in EC mediates vascular tone regulation under hypoxia and underlies the paradoxical difference in the response of pulmonary and systemic arterial SM to hypoxia. Our data suggest that HPV development in GPPA might, at least partially, be driven by EC depolarization spreading to the underlying SM cells.

Role of myocardium and endothelium in coronary vascular smooth muscle responses to hypoxia

Hypoxia triggers a mechanism that induces vasodilation in the whole heart but not necessarily in isolated coronary arteries. We therefore studied the role of cardiomyocytes (CM), smooth muscle cells (SMC), and endothelial cells (EC) in coronary responses to hypoxia (PO(2) of 5-10 mmHg). In an attempt to determine the factor(s) released in response to hypoxia, we inhibited the contribution of adenosine, ATP-sensitive K(+) channels, prostaglandins, and nitric oxide. Isolated rat septal artery segments without (-T) and with a layer of cardiac tissue (+T) were mounted in a double wire myograph, and constriction was induced. Hypoxia induced a decrease in isometric force of 21% and 61% in -T and +T segments, respectively (P < 0.05). EC removal increased the relaxation to hypoxia in -T segments to 33% but had the same effect in +T segments (61%). Only one of the inhibitors, the adenosine antagonist in +T segments, partially affected the relaxation due to hypoxia. The role of adenosine is thus limited and other mechanisms have to contribute. We conclude that hypoxia induces a relaxation of SMC that is augmented by the presence of CM and blunted by the endothelium. A single mediator does not induce those effects.