Aldosterone biosynthesis is ordinarily stimulated by reduced dietary sodium, heat exposure, exercise, and usually by increases in plasma renin activity although we have seen conditions where the two have been separated.
So I can only conclude that increased sodium and fluid reabsorption in the face of reduced dietary salt intake was the driving factor for those endocrinological responses.
Increased plasma renin activity was probably due to diminished splanchnic blood flow, as Rowell (Rowell, 1986) has shown; decreased plasma volume or reduced renal blood flow ordinarily elicits a very immediate response in terms of renal tubular biosynthesis and release of plasma renin. These endocrinological responses thus stimulate very efficient electrolytes and, thus, fluids.
PARTICIPANT: I was going to ask somewhat of a similar question. I was trying to figure out the mechanisms, first, when you kept everything constant, the aldosterone and renin went up. I was fascinated that they went back down.
DR. FRANCESCONI: They did and that could be a result of just the achievement of new steady states after the full acquisition of acclimation and expansion of plasma volume. It could he an increased sensitivity of both the sweat glands and the kidney cells to the activity of the PRA and the aldosterone.
Nothing that we saw would indicate that any of the subjects were becoming either hyponatremic or hypokalemic in these studies.
PARTICIPANT: As acclimation progressed, the intensity of these endocrine responses was moderated. Is that just due to increased sensitivity to PRA and ALDO.
DR. FRANCESCONI: That is just one of the mechanisms proposed.
PARTICIPANT: But, the mechanisms remain very vague.
DR. FRANCESCONI: As an aside, I have had rats on low sodium diets for up to 70 or 80 days with no real significant effects on circulating sodium levels and almost nonexistent urinary or salivary sodium.
PARTICIPANT: I am not so sure that is a change in sensitivity because in one of those slides that you showed where you looked at preacclimatization and postacclimatization, if you looked at the two lines there was a difference between the two, but those two lines looked like they were parallel. They appeared to be rising at the same rate for PRA and ALDO.
It looked as if the sensitivity was the same but just the initial values were different.
DR. FRANCESCONI: Actually, on day 1 of heat acclimation the responses