Preliminary evidence of extrarenal sodium storage in a large mammal: implications for comparative physiology and hypertension research

Abstract

Under conditions of dietary sodium (Na + ) excess, the kidneys may fail to adequately excrete Na + , potentially compromising blood pressure homeostasis. Body tissues, such as skin, can offer sites of short-term extrarenal Na + storage and previous research has shown that this can help guard against hypertension in small mammals (e.g., rodents). Large mammals have relatively greater Na + storage potential, but whether extrarenal Na + storage occurs for this group is unknown. Here, we report preliminary evidence of extrarenal Na + storage in cattle. We provided a large pulse-dose of NaCl to four cattle (body mass: ~720 kg) and measured excretion of Na + and potassium (K + ) in urine and faeces for a period of 7-days. Following NaCl administration, Na + excretion spiked in both urine and faeces for ~ 48 h before returning to baseline measurements. After ~ 96 h, however, Na + excretion increased again; a consistent physiological phenomenon across all individuals studied. We did not observe a pattern in urinary K + excretion, indicating that the mechanism of Na + storage does not appear to involve exchange for K + . However, faecal K + excretion was reciprocal to that of Na + , presumably reflecting exchange of Na + /K + across the walls of the large intestine. We infer that during the initial period of Na + stress, short-term extrarenal Na + storage occurred and the stored Na + was later released only when the body had returned to Na + homeostasis. Additional experiments are required to understand how patterns of Na + regulation changes across body sizes and the specific body compartments involved. Cattle may be a useful model system for examining the impact of high Na + intake in mammals larger than humans.

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