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.