Glucose-mediated inhibition of calcium-activated potassium channels limits alpha-cell calcium influx and glucagon secretion.

Pancreatic alpha-cells exhibit oscillations in cytosolic Ca2+ (Ca-c(2+)), which control pulsatile glucagon (GCG) secretion. However, the mechanisms that modulate alpha-cell Ca-c(2+) oscillations have not been elucidated. As beta-cell Ca-c(2-) oscillations are regulated in part by Ca2+-activated K+(K-SLOW) currents, this work investigated the role of K-SLOW, in alpha-cell Ca(2+ )handling and GCG secretion. alpha-Cells displayed K-SLOW, currents that were dependent on Ca2+ influx through L- and P/Q-type voltage-dependent Ca2+ channels (VDCCs) as well as Ca2+ released from endoplasmic reticulum stores. a-Cell K s i m ., was decreased by small-conductance Ca'activated K+ (SK) channel inhibitors apamin and UCL 1684. largeconductance Ca2+-activated K+ (BK) channel inhibitor iberiotoxin (IbTx), and intermediate-conductance Ca2+-activated K+ (BK) channel inhibitor TRAM 34. Moreover, partial inhibition of alpha-cell K-SLOW, with apamin depolarized membrane potential (V-m) (3.8 +/- 0.7 mV) and reduced action potential (AP) amplitude (10.4 +/- 1.9 mV). Although apamin transiently increased Ca2+ influx into a-cells at low glucose (42.9 +/- 10.6%), sustained SK (38.5 +/- 10.4%) or BK channel inhibition (31.0 +/- 11.7%) decreased alpha-cell Ca2+ influx. Total alpha-cell Ca-c(2+) was similarly reduced (28.3 +/- 11.1%) following prolonged treatment with high glucose, but it was not decreased further by SK or BK channel inhibition. Consistent with reduced alpha-cell Ca-c(2+) following prolonged K-SLOW, inhibition. apamin decreased GCG secretion from mouse (20.4 +/- 4.2%) and human (27.7 +/- 13.1%) islets at low glucose. These data demonstrate that K-SLOW, activation provides a hyperpolarizing influence on alpha-cell V-m that sustains Ca2+ entry during hypoglycemic conditions, presumably by preventing voltage-dependent inactivation of P/Q-type VDCCs. Thus, when alpha-cell Ca-c(2+) is elevated during secretagogue stimulation, K-SLOW activation helps to preserve GCG secretion.