Cardiovascular and sympathetic effects of 240-fold salt intake variations--studies in rats, with comparisons to man

Experiments are surveyed, suggesting that rats (WKY and SHR) have a very wide 'safety range' concerning salt intake with respect to cardiovascular homeostasis. Further, they are able to maintain their 'resting' volume and circulatory equilibria remarkably well over a 240-fold range of intakes, at least when young. The major longterm functional consequences of low and high intake were noted concerning sympathetic nervous control, chronic low Na intake leading to reduction of the transmitter release per impulse, and vice versa at high intakes. Otherwise blood volume, exchangeable sodium, cardiac and vascular muscle functions remained largely unchanged during rest, but the altered capacity of sympathetic control made the low Na rats dangerously vulnerable to e.g. fluid losses. In these respects SHR were, if anything, worse off than WKY. If allowed to choose their Na intake, the rats put themselves at an intake 30-50 times above that on which they can barely manage, and SHR on an even higher level than WKY. Some comparisons to man's situation are made, adjusting for differences in body size and metabolic rate. Such data suggest that human Na intakes in western societies are in fact similar to those instinctively chosen by e.g. sheep or rats, when on free access to salt. Some aspects of the assumed relationships between Na intake and primary hypertension, are also discussed. At least in the SHR variant of primary hypertension high salt intake is evidently of fairly subordinate pathogenetic importance, as seems to be the case also in the great majority of human subjects, to judge from recent epidemiological studies.