Evolution of control of epithelial transport in vertebrates.

In this review, the control of osmoregulation in the whole animal is discussed. Two main areas are considered: (i) the interaction between neural and hormonal control of osmoregulation and (ii) the relationship between vascular and osmoregulatory responses to these controlling factors. In the first area, the importance of neural influence on bullfrog kidney is described. The data suggest that alpha-adrenergic mechanisms are involved in glomerular as well as tubular functions. Innervation of tubular cels is seen in electron microscopic studies. Effects of neurotransmitters on fish gills and amphibian urinary bladder and skin transport are discussed. It is suggested that neural control in fish and amphibian epithelial transport might play an important role in osmoregulation. In the second area, a hypothesis is advanced for the evolution of renal function of arginine vasotocin (AVT). AVT probably first appeared as a vascular hormone. Its diuretic effect on fishes could be due to its systemic pressor action. During evolution, redistribution of vascular receptors, which became more numerous or more sensitive in the preglomerular circulation, resulted in glomerular antidiuresis as seen in the mudpuppy and bullfrog. Tubular receptors to AVT, present in the bullfrog, enhanced the antidiuretic property of this peptide. Effects of neurotransmitters on gill transport could be due in part to their vascular action on the branchial vasculature. The intimate relationship between vascular and osmoregulatory responses to controlling factors could be important in our understanding of the evolution of the control of osmoregulation.