Effects of temperature and oxygen availability on circulating catecholamines in the toad Bufo marinus

Abstract

The release of catecholamines during hypoxia has received limited attention in amphibians and the adrenergic regulation of cardio-pulmonary functions is, therefore, not well understood at the organismic level. To describe the changes in plasma catecholamine concentrations, we exposed toads ( Bufo marinus) to different levels of hypoxia at two temperatures (15 and 25°C). In addition, blood oxygen binding properties were determined in vitro at 15 and 25°C at two different pH values. Hypoxia elicited a significant increase in plasma catecholamines (adrenaline and noradrenaline) at both temperatures, in spite of a respiratory alkalosis. At 15°C, the increase was from 2.6±1.0 in normoxia to 4.8±1.4 ng ml −1 at an inspired oxygen fraction of 0.05. At 25°C, the hypoxic release of catecholamines was significantly higher (maximum levels of 44.8±11.6 ng ml −1). Plasma noradrenaline concentration was elevated at the most severe hypoxic levels, suggestive of an adrenal release. The arterial oxygen threshold for catecholamine release were approximately 1.0 mmol O 2 l −1 blood or a PaO 2 of 30 mmHg. The P 50 values at 15°C were 23.5±0.7 and 28.9±1.0 mmHg at pH 7.98±0.01 and 7.62±0.02, respectively, and increased to 36.5±0.6 and 43.0±1.1 mmHg at pH 8.04±0.04 and 7.67±0.05, respectively, at 25°C. The oxygen equilibrium curves were linear when transformed to Hill-plots and Hills n (the haemoglobin subunit co-operativity) ranged between 2.24 and 2.75. The in vitro blood O 2 binding properties corresponded well with in vivo data.