Regulation of coronary diameter by myogenic mechanisms in arterial microvessels greater than 100 microns in diameter.

We performed the present study to determine the quantitative significance of transient and steady-state myogenic responses in isolated coronary resistance vessels greater than 100 microns in diameter. Small coronary arteries were isolated from freshly excised porcine hearts (n = 14) and were studied under static conditions in a superfused vessel chamber that allowed internal diameter to be assessed continuously using video microscopic techniques. At a mean distending pressure of 50 mmHg, the passive diameter of resistance arteries following sodium nitroprusside averaged 181 +/- 10 (SE) microns. Upon heating, vessels developed spontaneous tone and constricted to 72 +/- 3% of their maximum diameter. After a 30-mmHg step change in coronary pressure from 50 to 80 mmHg, there was an initial dilation from 131 +/- 9 to 149 +/- 11 microns, followed by myogenic constriction that returned diameter to the initial value over a period of several minutes (134 +/- 10 microns, P = NS). Directionally opposite peak transients were observed during reductions in pressure. Steady-state diameter remained relatively constant over a pressure range from 100 to 20 mmHg. Although these data demonstrate that myogenic responses are present in resistance arteries that are greater than 100 microns in diameter, their contribution to steady-state coronary resistance changes as pressure is varied is small. This suggests that additional mechanisms contribute to the phenomenon of coronary autoregulation in this class of resistance vessel.