Disparate effects of substance P on systemic and coronary beds in conscious dogs

Previous studies in anesthetized animals indicated that substance P is a coronary and peripheral vasodilator. However, coronary vasodilation was only transient perhaps because of tachyphylaxis. In the present study, the steady-state effects of intravenous substance P on systemic and coronary beds were investigated in conscious, instrumented dogs.

With intact autonomic reflexes, 5 ng/kg/min i.v. substance P resulted in increases (p less than 0.01) in cardiac output by 22 +/- 5%, in decreases (p less than 0.01) in mean arterial pressure by 9 +/- 2%, and in total peripheral resistance by 23 +/- 4% 7-9 minutes after the beginning of substance P infusion. Heart rate increased (p less than 0.01) by 35 +/- 7% and left ventricular dP/dt (p less than 0.05) by 13 +/- 4%. In this situation, coronary blood flow decreased (p less than 0.01) by 19 +/- 4% and coronary vascular resistance increased (p less than 0.05) by 13 +/- 5%. Myocardial oxygen delivery was reduced (p less than 0.05) by 13 +/- 5% and the arteriovenous oxygen difference widened (p less than 0.01). After ganglionic blockade, increases in cardiac output, heart rate, and left ventricular dP/dt with substance P administration were abolished, but total peripheral resistance and mean arterial pressure decreased (p less than 0.01) by 12 +/- 3% respectively. Under these conditions, coronary blood flow decreased (p less than 0.01) by 37 +/- 5% and coronary vascular resistance increased (p less than 0.01) by 47 +/- 8%, which were more (p less than 0.01) than control responses. In this situation, myocardial oxygen delivery was reduced (p less than 0.01) by 37 +/- 4% and the arteriovenous oxygen difference widened (p less than 0.01). Intracoronary infusion of substance P (0.4 ng/kg/min) resulted in significant and sustained decreases in coronary blood flow, which were similar before and after ganglionic blockade.

Thus, in conscious dogs, systemic vasodilation is the prevailing effect of substance P, but paradoxically, this peptide simultaneously elicits coronary vasoconstriction.