AJP - Regulatory, Integrative and Comparative Physiology, Vol 256, Issue 1 187-R192, Copyright © 1989 by American Physiological Society
Direct cardiac stimulation by arginine vasotocin in bullfrogs (Rana catesbeiana)
J. S. Sham, W. H. Sawyer and P. K. Pang
Department of Environmental Health Sciences, Johns Hopkins Medical Institutions, Baltimore, Maryland 21205.
The antidiuretic and vasopressor effects of arginine vasotocin (AVT) in
bullfrogs have been well documented. However, the direct cardiac
stimulatory effects of AVT have not been previously reported. We found that
AVT stimulates significant increases in heart rate, pulse pressure, and the
maximal time derivative of ventricular pressure in anesthetized bullfrogs
after the beta-adrenergic and muscarinic receptors in the heart were
blocked by atropine and propranolol. In spontaneously beating isolated frog
atria, AVT increased the beating rate by 20% and the contractile force by
100%. The inotropic effect of AVT was also demonstrated in electrically
driven atria and ventricles. Propranolol was ineffective in blocking these
cardiac responses in vitro. The Hill coefficients of the
concentration-response curves did not differ from unity. In conclusion, AVT
possesses both positive chronotropic and inotropic effects in bullfrogs,
Rana catesbeiana, probably by direct stimulation of its specific receptors
in the heart.
