Effects of strontium ions on contraction and action potential in rabbit papillary muscles. A comparison with effects of tetraethylammonium ions

The effects of Sr2+ on contraction and action potential were studied in rab bit papillary muscles and compared with effects of tetraethylammonium (TEA( +)). The membrane potential was measured with KCl-filled microelectrodes an d the contraction was simultaneously recorded using a mechanoelectrical tra nsducer. A partial (90%) substitution of extracellular Ca2+ (Ca-e(2+)) by S r2+ produced stimulation frequency-dependent prolongation of the action pot ential (AP) with a dominant phase "plateau" as well as prolongation of the contraction. At low frequencies where the AP prolongation was well pronounc ed, the contraction became biphasic. The effect of Sr2+ on both AP and cont raction was blocked by nifedipine (10 mu mol/l) or by increasing Ca-e(2+). Ryanodine suppressed the early contraction component only. AP was prolonged to a similar extent and in the same frequency-dependent manner by TEA(+) ( 20 mmol/l). Despite similar AP configuration, no biphasic contraction devel oped in the presence of TEA(+). High Ca-e(2+) (10 mmol/l) or low Na-e(+) (7 0 mmol/l) suppressed the TEA(+) effect on AP. The data indicate that the tw o components of the biphasic contraction are of different origin; the early one is activated by activator cation released from the sarcoplasmic reticu lum while the late one results from the Sr2+ entry across the sarcolemma vi a L-type Ca2+ channels.