Low pHo boosts burst firing and catecholamine release by blocking TASK-1 and BK channels while preserving Cav1 channels in mouse chromaffin cells

Mouse chromaffin cells (MCCs) generate action potential (AP) firing that regulates the Ca2+-dependent release of catecholamines (CAs). Recent findings indicate that MCCs possess a variety of spontaneous firing modes that span from the common `tonic-irregular' to the less frequent `burst' firing. This latter is evident in a small fraction of MCCs but occurs regularly when Nav1.3/1.7 channels aremade less available or when the Slo1 beta 2-subunit responsible for BK channel inactivation is deleted. Burst firing causes large increases of Ca2+-entry and potentiates CA release by similar to 3.5-fold and thusmay be a key mechanism for regulatingMCCfunction. With the aim to uncover a physiological role for burst-firing we investigated the effects of acidosis onMCC activity. Lowering the extracellular pH(pHo) from7.4 to 7.0 and 6.6 induces cell depolarizations of 10-15mVthat generate repeated bursts. Bursts at pHo 6.6 lasted similar to 330 ms, occurred at 1-2 Hz and caused an similar to 7-fold increase of CA cumulative release. Burst firing originates fromthe inhibition of the pH-sensitive TASK-1/TASK-3 channels and from a 40% BK channel conductance reduction at pHo 7.0. The same pHo had little or no effect on Nav, Cav, Kv and SK channels that support AP firing in MCCs. Burst firing of pHo 6.6 could be mimicked by mixtures of the TASK-1 blocker A1899 (300 nM) and BK blocker paxilline (300 nM) and could be prevented by blocking L-type channels by adding 3 mu M nifedipine. Mixtures of the two blockers raised cumulative CA-secretion even more than lowpHo (similar to 12-fold), showing that the action of protons on vesicle release is mainly a result of the ionic conductance changes that increaseCa(2+)-entry during bursts. Our data provide direct evidence suggesting that MCCs respond to low pHo with sustained depolarization, burst firing and enhanced CA-secretion, thus mimicking the physiological response of CCs to acute acidosis and hyperkalaemia generated during heavy exercise and muscle fatigue.