Strontium-induced creep currents associated with tonic contractions in cardiac myocytes isolated from guinea-pigs.

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1. Strontium can replace calcium in a number of physiological and biochemical processes. The effects of Sr2+ were investigated in enzymatically isolated ventricular myocytes of the guinea-pig. Action potentials and membrane currents were measured with the patch-clamp technique used in the whole-cell recording configuration. Mechanical activity was assessed utilizing a laser-light diffraction system for sarcomere length measurements in single heart cells. 2. When experiments where carried out using 2 mM-Sr2+ to replace 2 mM-Ca2+ action potentials were found to be prolonged up to severalfold. Voltage-clamp experiments revealed that the slow inward current (Isi) inactivated more slowly. With Ca2+ replaced by Sr2+, the onset of the twitch was delayed, the maximum shortening was increased and a marked voltage-dependent tonic shortening developed. 3. Voltage-clamp pulses of 3.3 s duration were applied to investigate changes of the steady-state current-voltage relationship produced by replacing Ca2+ with Sr2+. Large slow changes of membrane currents produced by Sr2+ were observed. The identity and time course of these currents were investigated after blocking Isi and potassium currents pharmacologically. The remaining current had many of the characteristics of 'creep currents' (Eisner & Lederer, 1979; Hume & Uehara, 1986 a, b). The creep currents were found to be paralleled by changes of the intracellular Sr2+ concentration, as determined by tracking the sarcomere length during the accompanying tonic contractions. 4. The creep currents were suppressed by Ni2+ (2 mM), a finding that suggests that the Na+-Ca2+ exchanger may be responsible for producing these currents (Kimura, Miyamae & Noma, 1987). The question remains, however, whether the Na+-Ca2+ exchanger is responsible for generating the currents itself or whether it may influence another current source by changing the intracellular Sr2+ concentration. 5. To test the role of the Na+-Ca2+ exchanger in producing the creep currents, the reversal potential of the creep current was investigated. Simple voltage protocols were inadequate to distinguish between the two current sources. However, loading the cytosol with Sr2+ by means of a second pipette sealed to the same cell in the presence of Ni2+ as an inhibitor of the Na+-Ca2+ exchanger revealed difference currents compatible with a non-specific cationic channel activated by intracellular Sr2+ (Ehara, Noma & Ono, 1988). 6. In conclusion, the creep currents produced when Ca2+ is replaced by Sr2+ appear to arise from an increase of intracellular Sr2+ which activates a non-specific cation channel. A contribution from the Na+-Ca2+ exchanger can not be excluded.

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