Binding and bioactivity of ovine and porcine prolactins in porcine mammary tissue

Differential binding of homologous and heterologous prolactin was investigated in porcine mammary tissue. Specific binding of ovine prolactin to porcine mammary membranes or tissue slices was significantly greater than specific binding of the homologous porfine prolactin. Ovine prolactin was also more potent than porcine prolactin in stimulating proliferation of Nb₂ cells. In contrast, stimulation of glucose metabolism in porcine mammary explants by porcine prolactin was greater than that by ovine prolactin. Differences in specific binding were probably not due to damage during iodination, as low concentrations of iodinated prolactins were similar to unlabelled prolactins in their abilities to stimulate proliferation of Nb₂ cells. Furthermore, electrophoretic analysis of medium from binding reactions suggested that differences in specific binding were not due to proteolytic cleavage of the homologous prolactin into large (>10 kDa) fragments. These studies suggest that ovine prolactin either binds to sites in addition to the authentic lactogenic receptor in porcine mammary tissue or that a significantly higher affinity of ovine prolactin for the porcine lactogenic receptor has little effect on its biological activity.