RESUMO
Equilibrium binding of 125 I-insulin by lactating rat mammary gland acini was carried out at 4§C for 18 hours. Data were analysed as recommended by Scatchard. Published work on insulin receptors in other tissues generally shows Scatchard plots with an upward concavity. This is interpreted either as two receptor populations of different affinities or one population whose affinity declines with increasing saturation (negative co-operativity). Our data showed a small decline in number of mammary gland receptors when animals were deprived of circulating prolactin. Our Scatchard plots for control, 48-hr starved or prolactin-deprived animals also exhibited a 'shoulder' in the region between free insulin concentrations of 1.1 ng/ml and 2.1 ng/ml (physiological levels). This may indicate a sub-population of receptors exhibiting positive co-operativity (increasing affinity with increasing saturation). There is, to our knowledge, only one similar published report of positive co-operativity of insulin receptors - in rat hepatoma. We are trying to correlate these findings with the known autophosphorylation and aggregation behaviour of insulin receptors. If positive co-operativity of insulin receptors occurs in other tissues, this can have important implications for insulin action and aetiology of diabetes mellitus (AU)
