Acute reversible SERCA blockade facilitates or blocks exocytosis, respectively in mouse or bovine chromaffin cells

Abstract

Pre-blockade of the sarco-endoplasmic reticulum (ER) calcium ATPase (SERCA) with irreversible thapsigargin depresses exocytosis in adrenal bovine chromaffin cells (BCCs). Distinct expression of voltage-dependent Ca²⁺-channel subtypes and of the Ca²⁺-induced Ca²⁺ release (CICR) mechanism in BCCs versus mouse chromaffin cells (MCCs) has been described. We present a parallel study on the effects of the acute SERCA blockade with reversible cyclopizonic acid (CPA), to repeated pulsing with acetylcholine (ACh) at short (15 s) and long intervals (60 s) at 37 °C, allowing the monitoring of the initial size of a ready-release vesicle pool (RRP) and its depletion and recovery in subsequent stimuli. We found (i) strong depression of exocytosis upon ACh pulsing at 15-s intervals and slower depression at 60-s intervals in both cell types; (ii) facilitation of exocytosis upon acute SERCA inhibition, with back to depression upon CPA washout in MCCs; (iii) blockade of exocytosis upon acute SERCA inhibition and pronounced rebound of exocytosis upon CPA washout in BCCs; (iv) basal [Ca²⁺]_c elevation upon stimulation with ACh at short intervals (but not at long intervals) in both cell types; and (v) augmentation of basal [Ca²⁺]_c and inhibition of peak [Ca²⁺]_c amplitude upon CPA treatment in both cell types, with milder effects upon stimulation at 60-s intervals. These results are compatible with the view that while in MCCs the uptake of Ca²⁺ via SERCA contributes to the mitigation of physiological ACh triggered secretion, in BCCs the uptake of Ca²⁺ into the ER facilitates such responses likely potentiating a Ca²⁺-induced Ca²⁺ release mechanism. These drastic differences in the regulation of ACh-triggered secretion at 37 °C may help to understand different patterns of the regulation of exocytosis by the circulation of Ca²⁺ at a functional ER Ca²⁺ store.

Keywords: Bovine adrenal chromaffin cells; CICR; Calcium signaling; Cyclopiazonic acid; Exocytosis; Mouse adrenal chromaffin cells; SERCA.