Abstract
The contributions of the autonomic nervous system and the cardiac pacing cells in the development of heat-acclimation-induced bradycardia were analyzed, and the effect of heat acclimation on the chronotropic response of the heart to heat stress (40° C) was studied. Rats were acclimated at 34° C for 0, 5, 14, 30 and 60 days. Heart rate (HR) was measured in conscious animals, using chronic subcutaneous electrodes. Sympathetic and parasympathetic influences were studied by IP administration of 0.1 and 1 mg/100 g body weight atropine and propranolol respectively, while intrinsic HR (HRi) was measured following administration of both drugs simultaneously. The effects of carbamylcholine and norepinephrine on the beating rate of isolated rat atria were investigated to study pacemaker responsiveness to neutrotransmitters. Up to day 14 of heat acclimation, bradycardia was attained by tonic parasympathetic acceleration (18%) and temporal sympathetic withdrawal (0.8% on day 14), to compensate for the gradually augmented HRi (2.5% and 8% on days 5 and 14, respectively). Following long-term acclimation HRi declined below pre-acclimation rate. This was associated with resumed sympathetic activity (16% and 10% on days 30 and 60 respectively) while parasympathetic activity continued to be high (18%). Tachycardia, known to occur with severe uncontrolled body hyperthermia, was attenuated following heat acclimation by 42%. It was concluded that during the initial phase of heat acclimation bradycardia is achieved primarily by changes in autonomic influences, while following long-term acclimation, changes in the intrinsic properties of the pacing cells (HRi) and the autonomic system both play a role.
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Atias G, Gold S, Shmuel M, Somer H, Horowitz M, Sahar A (1988) Effects of long term heat exposure on the auditory nerve-brainstem evoked responses. J Therm Biol 13:175–177
Badeer HS (1975) Resting bradycardia of exercise training: A concept based on currently available data. In: Roy Paul-Emile, G. Rona (eds) The mebaolism of contraction. Recent advances in studies on cardiac structure and metabolism. University Park Press, Baltimore, pp 553–560
Bolter CP, Banister EW, Singh AK (1986) Intrinsic rates and adrenergic respopnses of atria from rats on sprinting, endurance and walking exercise programmes. Aust J Exp Biol Med Sci 64:251–256
Cavero I, Riggenbach H, Wall M, Gerold M (1976) Analysis of cardiac chronotropic responses to some autonomic blocking agents in conscious trained dogs. Eur J Pharmacol 39:193–202
Furuyama F, Ohara K, Ota A (1984) Estimation of rat thermoregulatory ability based on body temperature response to heat. J Appl Physiol Respir Environ Exercise Physiol 57:1271–1275
Gorman AJ, Proppe DW (1984) Mechanisms producing tachycardia in conscious baboons during environmental heat stress. J Appl Physiol Respir Environ Exercise Physiol 56:441–446
Gwosdow AR, Besh EL, Chen CL (1985) Acclimation of rats following stepwise or direct exposure to heat. J Appl Physiol Respir Environ Exercise Physiol 59:408–412
Horowitz M (1976) Acclimatization of rats to mild heat: Body water distribution and adaptability of submaxillary salivary gland. Pflügers Arch 366:173–176
Horowitz M (1990) Prolonged exposure to heat stress: acquired Physiological adaptations — cost and benefits. Archives of complex environmental studies. 2:11–14
Horowitz M, Meiri U (1985) Altered responsiveness to parasympathetic activation of submaxillary salivary gland in the heat acclimated rat. Comp Biochem Physiol [A] 80:57–60
Horowitz M, Meiri U (1985) Thermoregulatory activity in the rat: effects of hypohydration, hypovolemia and hypertonicity and their interaction with short-term heat acclimation. Comp Biochem Physiol [A] 82:577–582
Horowitz M, Samueloff S (1988) Cardiac output distribution in thermally dehydrated rodents. Am J Physiol 254:R109-R116
Horowitz M, Argov D, Mizrahi R (1983) Interrelationships between heat acclimation and salivary cooling mechanism in conscious rats. Comp Biochem Physiol [A] 74:945–949
Horowitz M, Meiri U, Oxenberg A (1986) Heat acclimation and heart rate: role of parasympathetic pathway. Proceedings of the International Union of Physiological Sciences, Vancouver
Kloog Y, Horowitz M, Meiri U, Galron R, Avron A (1985) Regulation of submaxillary gland muscarinic receptors during heat acclimation. Biochim Biophys Acta 845:428–435
Knowlton FP, Starling EH (1912) The influence of variations in temperature and blood pressure on the performance of the isolated mammalian heart. J Physiol (Lond) 44:206–219
Nobel D (1979) The initiation of the heart beat. Clarendon Press, Oxford
Nylander E, Sigvardsson K, Kilbom AA (1982) Training induced bradycardia and intrinsic heart rate in the rat. Eur J Appl Physiol 48:189–199
Oron Y, Falach O, Marmari I, Horowitz M (1989) Modulation in Rb and Cl efflux in submaxillary gland slices with the course of heat acclimation. Comp Biochem Physiol [A] 94:673–676
Rowell LB (1983) Human cardiovascular adjustment to heat stress. In: Shepherd, Abbound (eds) Handbook of Physiology, Cardiovascular system vol III. American Physiological Society, Bethesda, pp 967–1025
SAS/STAT (1989) Users Guide, vol II Cary, NC-SAS Inst. Inc. p 945
Walsh RR (1969) Heart rate and its neural regulation with rising body temperature in anesthetized rats. Am J Physiol 217:1139–1143
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Horowitz, M., Meiri, U. Central and peripheral contributions to control of heart rate during heat acclimation. Pflügers Arch. 422, 386–392 (1993). https://doi.org/10.1007/BF00374295
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DOI: https://doi.org/10.1007/BF00374295