Abstract
1. Rabbit's atria attached with functional sympathetic postganglionic nerves were used in the present study. Transmembrane potentials were recorded from single pacemaker fibers of the sino-atrial (S-A) node. 2. The maximal diastolic potential and the overshoot of action potentials varied inversely with [K+]0 and varied directly with [Ca++]0. The threshold potential was not affected by a rise in [Ca++]0. The depolarization time was a direct function of [K+]0. The rate of repolarization was not measurably affected by alterations of [K+]0. The early repolarization was slowed by a reduction of [Ca++]0 but accelerated by its elevation, whereas the late repolarization was accelerated by a reduction of [Ca++]0 but slowed by its elevation. 3. The steady state cycle length between pacemaker action potentials was increased in K+-poor, Ca++-poor and K+-rich solutions. The positive chronotropic response to sympathetic nerve stimulation was increased with decreasing [K+]0 and with increasing [Ca++]0. Magnesium ions increased the prestimulation cycle length, but did not affect considerably the positive chronotropic effect of the nerve stimulation. No changes in the prestimulation cycle length and the positive chronotropic effect were produced by an increase in osmotic pressure to 19.8 mM. 4. The gradient of slow depolarization during diastole was increased by cardiac noradrenaline, the increase being marked in K+-poor and Ca++-rich solutions. The action potential duration was increased by the amine at the highest [K+]0 used, but was decreased at low [Ca++]0. 5. It is suggested that the potentiation by a reduction of [K+]0 of the membrane effect and the positive chronotropic effect of sympathetic nerve stimulation results either from a potentiation of actions of cardiac noradrenaline on the K+-or Na+-permeability of the diastolic pacemaker membrane or from an inhibition of the uptake of noradrenaline by sympathetic nerve terminals. Modification of the effects of sympathetic nerve stimulation by alterations of [Ca++]0 results in large part from the amount of noradrenaline release.
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