Evolutionary biology of prolactins and somatotropins II. Electrophoretic comparison of tetrapod somatotropins

Abstract

Proteins of the adenohypophyses of representatives of all four classes of tetrapods were separated by polyacrylamide disc gel electrophoresis. These were tested for somatotropic activity in the toad, Bufo boreas, using increases in linear growth as the criterion for somatotropic activity. In all species examined, somatotropic activity was associated with prominent, slow-migrating adenohypophysial proteins. However, some prolactins also exhibit somatotropic activity. Ovine and bovine prolactins (NIH) had somatotropic activity that cannot be accounted for by growth hormone contamination: these prolactins are considerably less potent than mammalian growth hormone. In two turtles, Pseudemys scripta and Chrysemys picta, proteins which previously had been identified as prolactins were as potent as growth hormones with respect to their somatotropic activity in the Bufo. Thus, these chelonians contained two distinct and prominent adenohypophysial proteins with somatotropic activity. Prolactin from a toad ( Bufo marinus) may also have somatotropic activity but is less potent than the homologous growth hormone. Our results clearly indicate that in all tetrapod species examined, prolactins and somatotropins are separate molecular entities, the prolactin always having the higher electrophoretic mobility. However, the results with the turtles, toad, and with purified ovine and bovine prolactin indicate that, as in primates, adenohypophysial proteins may have both prolactin and somatotropic activities. Human placental lactogen (=chorionic somatomammotropin) was ineffective in the toad growth test. Estimation of the concentration of prolactin and growth hormone in adenohyphphyses by densitometry revealed that mammals had relatively high growth hormone levels in comparison to prolactin. Avian and reptilian glands showed lower concentrations of both hormones, with no appreciable differences between them. In amphibians, several species had unusually high prolactin levels and relatively low growth hormone concentrations: growth hormone could not be detected in the salamander ( Ambystoma tigrinum). Electrophoretic identification of somatotropins of these tetrapods should permit meaningful studies to be undertaken to elucidate the physiology of this hormone in nonmammalian species.