Effects of extracellular changes on spontaneous heart rate of normoxia- and anoxia-acclimated turtles (Trachemys scripta)

Abstract

Heart rate (f(H)) of the anoxia-tolerant freshwater turtle (Trachemys scripta) during prolonged anoxia exposure is 2.5- to 5-times lower than the normoxic rate, but whether alterations in blood composition that accompany prolonged anoxia contribute to this bradycardia is unknown. We examined how temperature acclimation, oxygen deprivation, acidosis, hyperkalemia, hypercalcemia and adrenaline affect chronotropy in the turtle myocardium. We monitored spontaneous contraction rates of right-atrial preparations obtained from 21 degrees C- and 5 degrees C-acclimated turtles that had been exposed to either normoxia or anoxia (6 h at 21 degrees C; 2 weeks at 5 degrees C). Sequential exposures to saline solutions were designed to mimic, in a step-wise manner, the shift from a normoxic to anoxic extracellular condition (for normoxia-acclimated preparations) or the reverse (for anoxia-acclimated preparations). Our results clearly show that prolonged anoxia exposure re-sets the intrinsic f(H) of turtles at both temperatures, with reductions in intrinsic f(H) in the range of 25%-53% compared with normoxia. This intrinsic change would contribute to the bradycardia observed with prolonged anoxia. Further, we found negative chronotropic effects of extracellular anoxia, acidosis and hyperkalemia, and positive chronotropic effects of hypercalcemia and adrenaline. The exact nature of these extracellular effects depended, however, on the acclimation temperature and the prior exposure of the animal to anoxia. With normoxia-acclimated preparations at 21 degrees C, combined anoxia and acidosis (pH reduced from approximately 7.8 to approximately 7.2) significantly reduced spontaneous f(H) by 22% and subsequent exposure to hyperkalemia (3.5 mmol l(-1)K(+)) further decreased f(H). These negative chronotropic effects were ameliorated by increasing the adrenaline concentration from the tonic level of 1 nmol l(-1) to 60 nmol l(-1). However, in anoxia-acclimated preparations at 21 degrees C, anoxia alone inhibited f(H) (by approximately 30%). This negative chronotropic effect was counteracted by both hypercalcemia (6 mmol l(-1) Ca(2+)) and adrenaline (60 nmol l(-1)). At 5 degrees C, only the combination of anoxia, acidosis (pH reduced from approximately 8.0 to approximately 7.5) and hyperkalemia (3.5 mmol l(-1) K(+)) significantly reduced spontaneous f(H) (by 23%) with preparations from normoxia-acclimated turtles. This negative chronotropic effect was fully reversed by hypercalcemia (10 mmol l(-1) Ca(2+)). By contrast, spontaneous f(H) of anoxia-acclimated preparations at 5 degrees C was not affected by any of the extracellular changes. We conclude that prior temperature and anoxia experiences are central to determining f(H) during prolonged anoxia in Trachemys scripta both as a result of the re-setting of pacemaker rhythm and through the potential influence of extracellular changes.