Intrarenal renin-angiotensin system in primitive vertebrates.

Teleost fishes, which have a simpler nephron structure and lack the macula densa, respond to lowered blood pressure by releasing renin. Inhibition of the angiotensin-converting enzyme decreases the resting level of blood pressure, suggesting that the RAS may participate in control of blood pressure in fish. Glomerulotubular balance is poorly developed, and GFR is readily increased by an increase in renal perfusion pressure. It is not clear at present whether angiotensin is involved physiologically in intermittency of glomerular filtration, or whether it controls GFR through its action on systemic blood pressure. Birds appear to have an intermediate form between primitive vertebrates and mammals in terms of morphologic structure of the JG apparatus and nephrons, and in renal function. Fowl, in which angiotensin causes biphasic depressor and pressor responses, do not respond to acute hypotension or hypovolemia by releasing renin unless blood pressure remains low. Unilateral infusion of hypertonic saline into the renal portal system, which perfuses the peritubular sinusoid, increases urinary excretion of sodium chloride in the infused kidney, accompanied by mild diuresis. The slight but significant decrease in PRA occurs after portal infusion of hypertonic saline. Further investigation will be necessary to determine on an individual nephron basis whether an increased tubular sodium or chloride load may alter GFR by a possible tubuloglomerular feedback mechanism.