In vivo adaptive control of beta-receptors and adenylate cyclase during short-term heat exposure in rat parotid glands

Heat acclimation and heat stress have different effects on cholinergic-induced calcium mobilization

There is evidence that the signal transduction array responsible for the secretion of water in evaporative cooling by the submaxillary gland of the rat is subject to heat acclimatory responses. The objectives of the present study were 1) to examine whether heat acclimation affects intracellular Ca2+ mobilization and, in turn, submaxillary glandular responsiveness; 2) to assess whether the acclimatory responses differ from those evoked on heat stress (HS). Experiments were conducted on submaxillary glands of rats acclimated at 34°C for 0, 2 [short-term heat acclimation (STHA)], and 30 [long-term heat acclimation (LTHA)] days. The resting cytosolic calcium concentration ([Ca2+]c) and the carbamylcholine-evoked calcium signal ([Ca2+]s) of dispersed glandular cells were measured using the fluorescent dye fura 2 AM. Inositol-1,4,5-trisphosphate (IP3)-sensitive endoplasmic reticulum Ca2+ stores were determined in permeabilized cells using fura 2 potassium salt. STHA resulted in a drop in both [Ca2+]s and IP3-sensitive Ca2+ stores. On LTHA, the [Ca2+]samplitude reverted to the preacclimation value, whereas the IP3-sensitive Ca2+ stores remained low. The drop in [Ca2+]s on STHA is in accord with the decreased glandular output (measured by 86Rb efflux) observed during this acclimation phase. However, after LTHA the enhanced glandular output despite reduced [Ca2+]s levels suggests an increased efficiency of cellular secretory mechanisms in that group. Collectively, the alterations in [Ca2+]ssupport our biphasic acclimation model (Horowitz M, Kaspler P, Marmari Y, and Oron Y. J Appl Physiol 80: 77–85, 1996.). In nonacclimated glands, HS caused an elevation in [Ca2+]s coincidentally with a decrease in the IP3 Ca2+ stores. In contrast, [Ca2+]s in both STHA and LTHA glands was not affected by HS, despite a marked increase in the IP3-sensitive Ca2+ stores in the LTHA glands. The opposing responses to HS and heat acclimation in calcium signaling and stores confirm the specificity of each process.