Manko B. Interdependence between the processes of respiration and Ca2+-signaling of pancreatic exocrine secretory cells

Mechanisms of regulation of rat pancreatic acinar cells respiration by Ca2+ were investigated. Respiration of digitonin-treated, unlike of intact, acini was substantially intensified by succinate or mixture of pyruvate, glutamate and malate. Low ADP concentration (100 mkM) did not influence the rate of oxygen uptake, whereas at higher concentration (750 mkM) brief intensification of respiration was observed when using nominally Ca2+-free medium, unlike of medium containing 100 nM Ca2+ (Ca2+-EGTA buffer). Apparent respiratory control indices were considerably higher in sucrose-based solution, suggesting a stronger coupling between respiration and oxidative phosphorylation. Sucrose-based medium, containing 100 nM Ca2+, is optimal for investigation of pancreatic mitochondria in situ respiration. The apparent constant of respiration half-activation of succinate, pyruvate or glutamate oxidation (K0.5) did not significantly change in the studied range of [Ca2+]. Maximum respiration rate Vmax at pyruvate oxidation increased due to increase of [Ca2+] from 10-7 to 5 x 10-7 or 10-6 M, respectively. At oxidation of succinate or glutamate Ca2+ did not significantly affect Vmax. Incubation of pancreatic acini with carbachol (1 and 10 mkM, 5 min) resulted in increase of respiration rate by 11.4 and 12.4 %, decrease of stored Ca2+ content by 33.0 and 29.4 % and 5- or 6.5-fold increase of amylase activity in the incubation medium, respectively. Addition of 10 mkM CCh into respiration chamber evoked biphasic stimulation of respiration. Rapid increase of respiration by 20.1% lasted for ~1 min, followed by decrease to level, by 11.5% higher than control. Addition of 1 mkM CCh caused monophasic increase by 11.5 %. Preincubation with 1 mkM CCh prior to cell permeabilization increased respiration rate at pyruvate+malate oxidation, but not at succinate oxidationThus, CCh (1 mkM) intensifies respiration and oxidative phosphorylation of acinar pancreacytes by feedforward mechanism via Ca2+-transport into mitochondria and activation of Ca2+/ADP-sensitive mitochondrial dehydrogenases. Prolonged action of 10 mkM CCh might impair mitochondrial functions.