Abstract
Direct perfusion of the sinus node artery under a constant pressure of 100mm Hg was arranged in eight canine hearts in situ. The administration of MnCl2, at doses from 10 μg to 10mg into the sinus node artery induced a longlasting negative chronotropic response. MnCl, blocked the effects of caffeine which induced double peaked positive chronotropic responses, i.e., initial rapid acceleration of sinus rate followed by slow and long-lasting acceleration. The initial acceleration was resistant to propranolol or tetrodotoxin, but completely blocked by MnCl2. Thus, the initial positive chronotropic response to caffeine may be probably ascribed to its essential effect of the movement of Ca ions on the SA nodal pacemaker cell.
