Age-dependent changes in electrophysiologic characteristics of fast and slow action potentials in rat papillary muscle.

Isolated papillary muscles from juvenile (about 2 months old, average weight of 250 g) and young adult rats (about 4 months old, average weight 485 g) were studied for age-dependent differences in the characteristics of fast and slow action potentials (APs). The fast and slow APs were recorded in 5.4 mM and 25 mM K+-Tyrode solutions, respectively (stimulation rate of 1 Hz). For the slow APs, the dose-response curves for isoproterenol versus Vmax (the maximum rate of rise of the APs), overshoot, and AP amplitude were linear between 10(-9) M and 10(-6) M (10(-5 M in some cases) in the juvenile and young adult rats. Isoproterenol pretreatment (1 mg/kg s.c., 1 h prior) decreased the slope of the dose-response curve, and saturation was achieved at a lower concentration. The Vmax, overshoot, and amplitude of both the fast and slow APs were somewhat smaller in the young adult rats than in the juvenile rats; there were no differences in the resting potential, AP duration, or threshold voltage. These results suggest that activation of a greater fraction of the beta-adrenergic receptors is coupled directly or indirectly to activation of a greater fraction of the slow channels. The pretreatment data suggest that down-regulation of the beta-adrenergic receptor may occur. The conductance per channel for the fast Na+ channels and slow channels, and (or) the number of both types of functional channels, may decrease with age.