Effects of racemic, dextro‐laevo‐propranolol and isoxuprine on the metabolic and endocrine response to cold in the newborn rabbit

Abstract

Abstract. The effect of racemic, dextro‐, laevo‐propranol or isoxuprine on the plasma glucose, free fatty acid (FFA), insulin, glucagon and glucocorticoid concentration was studied in rabbits aged 3–5 days at 35°C and after exposure to 25°C for 3 h. Cold exposure alone caused a large rise in plasma FFA and a significant rise in plasma insulin and glucocorticoids. At 35°C laevo‐propranolol depressed both plasma FFA and glucose where‐as dextro‐propranolol caused a significant rise in plasma glucose, FFA, insulin and glucagon and a fall in plasma glucocorticoid concentration. Racemic propranolol resembled the laevo‐isomer in depressing plasma FFA and the dextro‐isomer in stimulating plasma insulin and glucagon but had no effect on the mean plasma glucose concentration. Isoxuprine caused a rise in plasma FFA and insulin levels. At 25°C laevo‐propranolol blocked the cold‐induced rise in plasma FFA, caused a rise in plasma insulin and an insignificant drop in plasma glucose. The effects of dextro‐propranolol seen at 35°C were abolished by exposure to 25°C. Racemic propranolol resembled the laevo‐isomer in partially blocking the rise in plasma FFA. Isoxuprine exaggerated the rise in plasma FFA and caused a significant fall in plasma glucagon concentration. Incubation of pieces of pancreas in vitro from rabbits aged 3–5 days or 5–7 weeks with the three types of propranolol at concentrations of 0.2, 2.0 and 20.0 μg/ml had no effect on basal or glucose stimulated insulin secretion. Racemic propranolol at concentrations of 40 and 60 μg/ml and laevo‐propranolol at 80 μg/ml inhibited insulin release stimulated by glucose from pancreas of 5–7 week rabbits in vitro.