The aim of the study was to determine the effect of hypothermia on I125 - iodoinsulin binding to heart plasma membranes. The membranes were prepared from control, normothermic rats (Tr = 35.6±0.2¡C) and hypothermic rats (Tr = 26.0±0.5¡C) and purified according to the method described by Havrankowa. In order to determine whether any decrease in blood insulin concentration could be related to changes in the number or in the affinity of insulin receptor sites according to the theory of 'down and up regulation' the binding assay was coupled with measurements of the plasma immunoreactive insulin levels. Scatchard analysis was used to determin the kinetic parameters of the hormone - receptor interaction. The data showed significant differences in the number and in the affinity of the binding sites, mainly in the class of low-affinity insulin receptors (LAIRs). The number of LAIRs in hypothermic hearts decreased by about 85% as compared with the number of LAIRs in normothermic or control hearts (expressed as 100%). By contrast, the values of Ka1 x R1 and of Ka2 x R2, which describe the functional features of insulin receptors, did not differ significantly in the hearts of normothermic and hypothermic rats. This may indicate that there is no functional difference in the overall ability of heart insulin receptors to bind insulin in hypothermic conditions in comparison to normothermia. Finally the suggestion that an inverse relationship exists between the number of receptors at target tissues and the concentration of circulating insulin is not consistent with our findings for the hypothermic heart.

Keywords: Rat, Hypothermia, heart muscle, insulin receptor