Interactive effects of variations in [Na]o and [Ca]o on rat atrial spontaneous frequency

The effects of varying the extracellular concentrations of Na and Ca ([Na]o and [Ca]o) on both, the spontaneous beating and the negative chronotropic action of verapamil, were studied in the isolated rat atria. Basal frequency (BF) evaluated by surface electrogram was 223 +/- 4 beats/min. in control Krebs-Ringer containing 137 mM Na and 1.35 mM Ca (N). It decreased by 16 +/- 3% by lowering [Na]o to 78 mM (LNa), 23 +/- 2% by lowering simultaneously [Na]o to 78 mM and [Ca]o to 0.675 mM (LNa+LCa) and 31 +/- 5% by lowering [Na]o to 78 mM plus increasing [Ca]o to 3.6 mM (LNa+HCa). At normal [Na]o, decrease (0.675 mM) or increase (3.6 mM) of [Ca]o did not modify BF; a reduction of ten times (0.135 mM of normal [Ca]o was effective to reduce BF by 40 +/- 13%. All negative chronotropic effects were BF-dependent. Dose-dependent bradycardia induced by verapamil was potentiated by LNa, LCa, and HCa. Independent but not additive effects of Na and Ca are shown by decreases in the values of [verapamil]o needed to reduce BF by 30% (IC30) with the following order of inhibitory potency: LNa > LCa > HCa > N, resulting LNa+HCa similar to LNa. The [verapamil]o that arrested atrial beating (AC) was also potentiated with the order LNa = LNa+LCa = LNa+HCa = LCa > HCa = N. The results indicate that rat atrial spontaneous beating is more dependent on [Na]o than on [Ca]o in a range of +/- 50% of their normal concentration. Also the enhancement of verapamil effects on atrial beating was more pronounced at LNa than at LCa.(ABSTRACT TRUNCATED AT 250 WORDS)