HEAT ACCLIMATION AND HYPOHYDRATION IN AGED RATS - THE INVOLVEMENT OF ADRENERGIC PATHWAYS IN THERMAL-INDUCED VASOMOTOR RESPONSES IN THE PORTAL CIRCULATION

1. The beneficial effects of heat acclimation on thermal induced vasom otor responses of hypohydrated aged rats were assessed by measuring th e isometric tension of aortic and portal rings of old and young rats u nder heat acclimation and hypohydration in response to alpha-adrenergi c (alpha-AR) and beta-adrenergic (beta-AR) stimulation (phenylephrine 10(-9)-10(-2) mM and isoprenaline 10(-9)-10(-4) respectively). In para llel, portal blood flow (PBF), which drains the splanchnic vasculature , was measured in conscious rats, before and during heat stress (42 de grees C). 2. In the aorta, heat acclimation augmented phenylephrine (a -AR) induced tension, to a great extent in the older rats. Hypohydrati on increased alpha-AR sensitivity in all experimental groups. Acclimat ion and aging brought about decreased responsiveness in isoprenaline i nduced relaxation (beta-AR) in both the aorta and the portal vein. Hyp ohydration increased beta-AR responsiveness in the portal vein of OR, acclimated and acclimated-hypohydrated rats. 3. Normothermic euhydrate d resting PBF was similar for young and old rats. Hypohydration decrea sed resting PBF. Upon heat stress, thermal induced vasoconstriction in hypohydrated YR and OR occurred earlier than in the euhydrated groups and was more pronounced. The latter responses were attenuated in the old rats. 4. Taken together, these results imply that chronic environm ental stressors such as heat acclimation and hypohydration produce sel ective alterations in AR responsiveness of the vasculature in both you ng and old rats. Consequently, thermoregulatory vasomotor mediated mec hanisms, as exhibited in this study in PBF, may differ in their respon siveness in these two age groups. Copyright (C) 1996 Elsevier Science Ltd.