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KW Hewett and MR Rosen
We used standard microelectrode techniques to study developmental changes in the transmembrane potentials of sinus nodes from 1- to 10- day-old and adult rabbits superfused with Tyrode's solution at 37 degrees C. Maximum diastolic potential did not change with age, but action potential amplitude in day 1 to 3 hearts was lower than that in adults. Sinus cycle length (SCL) also was shorter in the young sinus nodes. In experiments on beta adrenergic stimulation the reduction of SCL in the young occurred at lower isoproterenol concentrations than in the adult, but the maximum reduction in SCL was the same at both ages (- 40%). The alpha agonist, phenylephrine, had no age-dependent effect on SCL or action potential characteristics in either age group. In addition, the inhibitory effect of alpha stimulation demonstrated previously in automatic fibers of the ventricular specialized conducting system and atrium did not occur in sinus node. Studies of Purkinje fibers demonstrated this inhibitory effect to be voltage- dependent; it did not occur at membrane potentials in the range of those in the sinus node. Our results suggest that whereas developmental changes in the response to beta adrenergic stimulation are demonstrable in the sinus node, there are no such changes in the response to alpha adrenergic stimulation.
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