Double peaked positive chronotropic response of the isolated blood-perfused S-A node to caffeine.

peaked positive chronotropic responses, i.e., rapidly induced initial acceleration followed by slowly induced but long-lasting one. The initial positive response to caffeine was hardly suppressd by either adrenergic beta-blocking agents, H 56/28 (alprenolol) and propranolol, or tetrodotoxin. On the contrary, the secondary positive chronotropic response was effectively inhibited by either adrenergic beta-blocking agents or tetrodotoxin. These results suggest that caffeine-induced initial acceleration is not due to catecholamine while the secondary one is due to catecholamine which is released by excitation of local adrenergic fibers. isolated S-A node; a cross circu lation; catecholamine; H 56/28; tetrodotoxin