Renin response to hemorrhage and hypotension in the aglomerular toadfish Opsanus tau

Renal renin and the juxtaglomerular cells evolved in primitive bony fishes, whereas the macula densa emerged later in vertebrate phylogeny. We attempted to determine whether a renal arteriolar baroreceptor exists in the toadfish Opsanus tau, which possess renin and granulated cells in the kidneys, but lack glomeruli and macula densa. Cumulative hemorrhage of 1.5, 3, 6, 12, and 18 ml/kg, or a single massive bleeding from unanesthetized toadfish, kept in 50% seawater, caused an immediate and significant decrease in mean aortic pressure and stepwise increases (5-20 times) of plasma renin activity (PRA). Papaverine (10 mg/kg) caused hypotension and increased PRA. Minoxidil (6-12 mg/kg) neither decreased blood pressure nor increased PRA. The results suggest that toadfish respond to hemorrhage and acute hypotension with renin release despite the absence of a macula densa. It remains to be determined whether decreased renal perfusion pressure due to decreased dorsal aortic pressure stimulated the receptor in the granulated cells or whether the renal nerves may be involved.