Regulation of potassium currents in the sino-atrial node

Regulation of potassium channels by cytosolic calcium and other factors was studied in guinea-pig isolated sino-atrial node cells. Rapidly activating I_KR and slowly activating I_KS were identified on the basis of biophysical factors (range of activating depolarisations, time of activation, rectification) and selective blockers (E4031 to suppress I_KR). The protein channels carrying I_KR and I_KS were localised to the sarcolemmal membrane using specific antibodies in immunohistochemical experiments. Loading cells with a calcium chelator (BAPTA) caused substantial reduction of I_KR and I_KS. Both currents were also reduced by the CAMK inhibitor KN- 93. It therefore appeared that cytosolic calcium plays a role in regulating I_KR and I_KS and that this effect was mediated at least in part through CAMK. AC may also be calcium sensitive, and production of cAMP and PKA activity may therefore be calcium dependent. cAMP appeared to regulate pacemaking; basal AC turnover is though to be high, and elevated cAMP appear to augment I_KR via PKA activity. Upon cAMP elevation (via AC stimulation or isoprenaline) I_KS augmentation was greater than I_KR. Agents that interfered with the function of the sarcoplasmic reticulum (ryanodine and cyclopiazonic acid) also reduced I_KR and I_KS , but to a lesser extent than BAPTA-loading or KN-93, and these observations indicate that calcium from the SR contributed partially to (was not the sole source) the calcium regulating I_KR and I_KS . The effects of the cytosolic BAPTA and of the other agents listed above were reinvestigated after stimulation of beta-adrenoceptor pathways with isoprenaline, and it was concluded that increased calcium entry and rises in cytosolic calcium play a significant role in mediating the effects of beta-adrenoceptor stimulation on I_KR and I_KS. Inhibitors with differential specificity for protein kinase subtypes showed a range of inhibitory effects, reflecting complex interactions between the different protein kinases upon I_KR and I_KS.