Delayed rectifier potassium currents, rapidly activating I_Kr and slowly activating I_Ks, play a role in cardiac repolarization, and deactivation of channels carrying these currents contributes to pacemaker activity in the sino-atrial node. Possible influence of cytosolic Ca²⁺ on these currents was investigated in guinea-pig isolated sino-atrial (SA) node cells.

SA node cells were isolated from hearts removed from guinea-pigs following stunning and cervical dislocation. Cells were superfused with a physiological salt solution containing 2.5 mM Ca²⁺ at 36 °C. Cells were voltage-clamped using a perforated-patch clamp technique. All currents were activated from a holding potential of -40 mV, I_K was measured using tail currents, and values stated as a mean ± S.E.M. % of the maximum control current at 700 ms. Under these conditions, currents after 40 ms depolarisations reflect predominantly I_Kr, while I_Ks contributes substantially to currents after 700 ms depolarisations. Statistical significance was evaluated using Student’s paired t test.

BAPTA (10 mM) suppressed I_K at both 40 ms (7 ± 2 vs. 23 ± 5% P < 0.05; n = 6) and 700 ms (28 ± 5 vs. 100 ±% P < 0.05; n = 6) duration depolarisations (to 40 mV). Nifedipine (5 mM) reduced I_K amplitude at both 40 ms (12 ± 1 vs. 17 ± 2% P < 0.05; n = 11) and 700 ms (85 ± 5 vs. 100 ± 0% P < 0.05; n = 11). Ryanodine (5 mM) also reduced I_K, but only after long duration depolarisations: 700 ms (82 ± 7 vs. 100 ± 0% P < 0.05; n = 9). KN93 (an inhibitor of calmodulin-dependent kinase) reduced current at 40 ms from 27 to 12 % and at 700 ms from 100 to 45 %. Isoprenaline (100 nM) increased I_K amplitude from 22 ± 6 to 37 ± 7 % at 40 ms (P < 0.05; n = 9) and 100 ± 0 to 197 ± 16 % at 700 ms (P < 0.05; n = 9); however, when SR Ca²⁺ release was inhibited with ryanodine, isoprenaline only increased the I_K amplitude of the longer duration depolarisation: 100 ± 0 to 132 ± 8 % at 700 ms (P < 0.05; n = 5).

These observations are consistent with an influence of cytsolic Ca²⁺ on delayed rectifier potassium currents. Buffering Ca²⁺ with BAPTA appeared to influence both components, as did blocking Ca²⁺ entry with nifedipine. Block of ryanodine-sensitive release channels in the SR appeared to affect predominantly I_Ks. Isoprenaline actions on I_K appear to be influenced by SR Ca²⁺. These actions might be in part be mediated through a kinase mechanism since the inhibitor KN-93 appeared to reduce both I_Kr and I_Ks, though further experiments are needed to test these possibilities.

This work was funded by the British Heart Foundation.

All procedures accord with current UK legislation.