Parietal Cell Volume Regulation During Acid Secretion

Abstract

We have previously shown that concomitant Na+/H+ and Cl-/HCO3 - exchange activation occurs during stimulation of acid secretion in cultured rabbit parietal cells, possibly related to a necessity for volume regulation during the secretory process. We now investigated whether cytoplasmic volume changes occur during secretagogue stimulation of cultured rabbit parietal cells, and the activation of which ion transport mechanisms is responsible for these changes. Cells were loaded with the fluorescent dye calcein and the calcein concentration within a defined cytoplasmic volume was recorded by confocal microscopy. 10–5M forskolin, l0–4M carbachol and hyperosmolarity (400 mosmol) resulted in a rapid increase in the cytoplasmic dye concentration, indicative of cell shrinkage, followed by recovery to baseline within several minutes, indicative of regulatory volume increase (RVI). We hypothesized that the initial shrinkage, because it was more rapid than fluid accumulation in the secretory vesicles, may be due to the opening of agonist-dependent basolateral or apical K+ or anion channels, and tested the effect of a variety of K+ and CI- channel inhibitors. The results can be summarized as follows: Ba2+-sensitive K+-channels, likely in the basolateral membrane, are constitutively open and their blockade results in rapid cell swelling and the prevention of forskolin- or carbachol-induced cytoplasmic shrinkage. The inhibition of agonist-activated Hoe 293B- and ChTX-sensitive and NPPB-sensitive anion channels, which are possibly apically located, strongly decreases agonist-activated shrinkage. Na+/H+ exchange inhibitors slightly reduced the initial cell shrinkage and significantly slowed the RVI, whereas 100µM bumetanide had no significant effect on either parameter.