Resting and maximal heart rates in ectothermic vertebrates

Resting and maximal heart rates (HR) in ectothermic vertebrates are general ly lower than those in endotherms and vary by more than an order of magnitu de interspecifically. Variation of HR transcends phylogeny and is influence d by numerous factors including temperature, activity, gas exchange, intrac ardiac shunts, pH, posture, and reflexogenic regulation of blood pressure. The characteristic resting HR is rarely the intrinsic rate of the pacemaker , which is primarily modulated by cholinergic inhibition and adrenergic exc itation in most species. Neuropeptides also appear to be involved in cardia c regulation, although their role is not well understood. The principal det erminants of resting HR include temperature, metabolic rate and hemodynamic requirements. Maximal HRs generally do not exceed 120 b min(-1), but notab le exceptions include the heterothermic tuna and small reptiles having HRs in excess of 300 b min(-1) at higher body temperatures. Temperature affects the intrinsic pacemaker rate as well as the relative influence of adrenerg ic and cholinergic modulation. It also influences the evolved capability to increase HR, with maximal cardiac responses matched to preferred body temp eratures in some species. Additional factors either facilitate or limit the maximal level of HR, including: (1) characteristics of the pacemaker poten tial; (2) development of sarcoplasmic reticulum as a calcium store in excit ation-contraction coupling; (3) low-resistance coupling of myocardial cells ; (4) limitations of force development imposed by rate changes; (5) efficac y of sympathetic modulation; and (6) development of coronary circulation to enhance oxygen delivery to myocardium. In evolutionary terms, both hemodyn amic and oxygen requirements appear to have been key selection pressures fo r rapid cardiac rates. (C) 1999 Elsevier Science Inc. All rights reserved.