Effects of reversible SERCA inhibition on catecholamine exocytosis and intracellular [Ca2+] signaling in chromaffin cells from normotensive Wistar Kyoto rats and Spontaneously Hypertensive rats

Abstract

Intracellular [Ca2+]c signaling and catecholamine (CA) exocytosis from adrenal chromaffin cells (CCs) differ between mammalian species. These differences partly result from the different contributions of Ca2+-induced Ca2+-release (CICR) from internal stores, which boosts intracellular Ca2+ signals. Transient inhibition of the smooth endoplasmic reticulum (SERCA) Ca2+ pump with cyclopiazonic acid (CPA) reduces CICR. Martínez-Ramírez et al. authors found that CPA had opposite effects on catecholamine secretion and intracellular Ca2+ signals in mouse and bovine CCs, where it enhanced and inhibited exocytosis, respectively. After CPA withdrawal, exocytosis diminished in mouse CCs and increased in bovine CCs. These differences can be explained if mouse CCs have weak CICR and strong Ca2+ uptake, and the reverse is true for bovine CCs. Counterintuitively, CPA reduced the amplitude of Ca2+ signals in both mouse and bovine CCs. Here we examined the effects of CPA on stimulated CA exocytosis and Ca2+ signaling in rat CCs and investigated if it alters differently the responses of CCs from normotensive (WKY) or hypertensive (SHR) rats, which differ in the strength of their CICR. Our results demonstrate that CPA application inhibits the voltage-gated exocytosis and Ca2+ transients in rat CCs, regardless of strain (SHR or WKY). CPA inhibited Ca2+ signals significantly more in SHR CCs than in WKY CCs. Despite the greater phylogenetic distance from the most recent common ancestors, CPA alters the CA secretion in rat CCs more similarly to bovine CCs than mouse CCs. Rat CCs display Ca2+ signals with one or two components. In CCs with two Ca2+ components, simultaneous amperometry also shows two bursts of exocytosis. The late component is abolished with CPA and Ryanodine, suggesting that it is due to CICR. Agents such as CPA that inhibit the SERCA pump and suppress catecholamine secretion indiscriminately from WKY and SHR in CCs are not likely helpful as therapeutic agents for hypertension.