Responses to amrinone in isolated cardiac muscles from cat, rabbit, and guinea pig

Amrinone (10(-5)-10(-2) M) produced a positive inotropic response in several electrically stimulated, isolated cardiac muscle preparations. The order of potencies for amrinone in the tissues studied was: cat papillary muscle greater than rabbit papillary muscle greater than muscle strips from rabbit left atria = guinea pig papillary muscle greater than guinea pig left atria. Amrinone also restored contractility to cat and rabbit papillary muscles which had been partially depolarized with 20 mM K+. The beta-adrenergic antagonists timolol and propranolol partially inhibited these responses in rabbit but not cat papillary muscles. Reserpine pretreatment did not influence the responses to amrinone in any of the preparations studied. In conclusion, (a) the cat papillary muscle was the most sensitive preparation to the inotropic effect of amrinone, and (b) in the rabbit papillary muscle, beta-adrenoceptors contribute to the amrinone response but are not solely responsible for the enhanced inotropy.