EVIDENCE FOR CONTRIBUTION OF EFFECTOR ORGAN CELLULAR-RESPONSES TO THEBIPHASIC DYNAMICS OF HEAT ACCLIMATION

The involvement of cellular processes in the biphasic dynamics of heat acclimation was studied. Key steps in the cholinergic signal transduc tion pathway for water secretion were measured in the submaxillary gla nd of acclimating [2-day short-term heat acclimation (STHA) and 30-day long-term heat acclimation (LTHA) at 34 degrees C] or acute heat-stre ssed (2 h at 40 degrees C) rats in vitro. Both the carbamylcholine (CC h)-induced maximal fractional rate and the total Rb-86(+) efflux, refl ecting K+ efflux and water transport, transiently decreased in STHA (P < 0.001). In LTHA, the total K+ efflux increased (P < 0.001), whereas the maximal fractional rate of efflux increased only slightly. During STHA, the density of the high-affinity binding site of the muscarinic receptors (MRs) increased by 50% and their affinity for the muscarini c antagonist [H-3]-N-methylscopolamine decreased transiently by 87%. B asal cytosolic Ca2+ concentration ([Ca2+](i)) decreased (P < 0.05), bu t the peak CCh-induced [Ca2+](i) increase resembled the control values . In LTHA, MR density continued to increase (100%; P < 0.05), whereas affinity resumed control values. Basal and CCh-induced [Ca2+](i) incre ases returned to control levels. We conclude that glandular cellular p rocesses follow a biphasic pattern with major apparent changes attribu table to events distal to the [Ca2+](i) rise. This was further validat ed by employing heat stress, which produced qualitatively different ef fects on the MR profile with a decrease in Rb-86(+) efflux comparable to STHA. Hence, although heat-induced changes in the proximal componen ts of the signal transduction pathway may contribute to altered regula tory span, the predominant apparent cellular effect is on the distal p art of the pathway.