Abstract
The sinus node artery was perfused in situ in 12 dogs. Selective administration of caffeine into the sinus node artery produced double peaked positive chronotropic responses, i.e., initial rapid acceleration of the sinus rhythm followed by slow but long-lasting acceleration. The initial positive chronotropic response to caffeine was not suppressed by treatment with a β-adrenergic blocking agent, propranolol or H 56/28, or by treatment with tetrodotoxin whereas these substances effectively blocked the secondary positive chronotropic response.It is suggested that the initial positive chronotropic response to caffeine is a direct effect on the S-A node and the secondary positive response is due to catecholamine release from adrenergic nerve terminals by excitation of nerve fibers.
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