Dissociation of subsarcolemmal from global cytosolic [Ca2+] in myocytes from guinea-pig coronary artery

1. Changes in cytosolic Ca2+ concentration (Delta[Ca2+](c)) were measured by global indo-1 fluorescence and compared with changes in subsarcolemmal Ca2+ concentration (Delta[Ca2+](sl)) indicated by Ca2+-activated K+ currents (I-K(Ca)). 2. At -50 mV holding potential, 10 mM caffeine increased both I-K(Ca) and [Ca2+](c) without measurable delay. While I-K(Ca) peaked within 0.3 +/- 0.16 s (mean +/- S.D.) and decayed to 50% within 0.4 +/- 0.2 s, Delta[Ca2+](c) peaked within 1.5 +/- 0.5 s and decayed to 50% within 5.2 +/- 1.0 s. The different time courses support the idea that [Ca2+](sl) and [Ca2+](c) deviate. 3. When 10 mM caffeine was applied 20 s after an initial 2 s caffeine application, I-K(Ca) was suppressed to 22 +/- 5% and Delta[Ca2+](c) to 40 +/- 4%. During the following 1 min caffeine-free period, I-K(Ca) recovered to 61 +/- 7% while Delta[Ca2+](c) remained at 40 +/- 3%. The differences between I-K(Ca) and Delta[Ca2+](c) suggest that Ca2+ deprivation and Ca2+ refilling is faster in peripheral than in central sarcoplasmic reticulum (SR). 4. During the loading period of indo-1, a spontaneous Delta[Ca2+](c) of 30-80 nM appeared both at -50 mV and at more positive potentials. The amplitude of spontaneous Delta[Ca2+](c) increased with the amplitude, the frequency or the fusion of spontaneous transient outward currents (STOCs). 5. Block of sarcolemmal Ca2+ fluxes by 1 mM La3+ increased [Ca2+](c) by 250 +/- 100 nM and suppressed the spontaneous Delta[Ca2+](c). However, La3+ did not significantly retard the rate of decay of STOCs which may therefore be limited by Ca2+ diffusion into the cytosol and not by Ca2+ extrusion. 6. The dissociation of I-K(Ca) (or STOCs) and Delta[Ca2+](c) may indicate a Ca2+ concentration gradient during Ca2+ release directed from the sarcolemma towards the centre of the cell, which later reverses direction.