AJP - Heart and Circulatory Physiology, Vol 243, Issue 1 94-H98, Copyright © 1982 by American Physiological Society
Effects of alpha-adrenergic receptor blockade on coronary circulation in conscious dogs
P. Macho, T. H. Hintze and S. F. Vatner
The effects of three alpha-adrenergic-receptor blocking agents
(phentolamine, prazosin, and trimazosin) were compared on the coronary
circulation and left ventricular (LV) function in chronically instrumented
conscious dogs. The three alpha-adrenergic-receptor blocking agents were
administered in equidepressor doses (mean arterial pressure fell by
approximately 20%) and in the presence of beta-adrenergic-receptor blockade
and constant heart rate. LV systolic pressure, LV end-diastolic pressure,
and LV end-diastolic diameter also fell similarly with the three drugs.
Phentolamine decreased the time rate of change of LV pressure (LV dP/dt) by
21 +/- 3%, whereas trimazosin and prazosin decreased LV dP/dt only by 14
+/- 2 and 11 +/- 2%, respectively. LV velocity was not changed with
trimazosin and prazosin but decreased with phentolamine by 12 +/- 4%. The
three drugs exerted differential effects on the coronary circulation. Only
trimazosin increased coronary blood flow (18 +/- 5%). Trimazosin decreased
late diastolic coronary resistance (LDCR) by 35 +/- 2%, which was
significantly more than reductions in LDCR induced by prazosin (22 +/- 2%)
and by phentolamine (11 +/- 3%). A test dose of phenylephrine (5.0
micrograms/kg) increased mean arterial pressure by 53 +/- 3.5 mmHg. After
trimazosin, prazosin, and phentolamine, the same dose of phenylephrine
increased mean arterial pressure by 24 +/- 2.1, 14 +/- 1.6, and 1.9 +/- 0.6
mmHg, respectively. The response after phentolamine was significantly less
than with trimazosin (P less than 0.01) and prazosin (P less than 0.02).
Thus the capacity of these three alpha-adrenergic-receptor blocking drugs
to dilate coronary vessels is inversely proportional to their capability to
block exogenous alpha-adrenergic-receptor agonists.
