Renal Concentrating Ability and Calcium Load Handling in the Spontaneously Hypertensive Rate

The renal response to an oral load of calcium and the renal concentrating ability of spontaneously hypertensive (SH) and normotensive (WKY) rats were examined. The response to calcium loading was similar between SH and WKY animals in terms of urine volume, osmolality and calcium excretion. Compared to WKY controls, the ability of SH rats to concentrate urine during dehydration was decreased. The injection of vasopressin had no effect on the urine output of water-loaded SH rats, although a similar injection returned urine flow to control levels in WKY animals. It is suggested that the decreased ability of the SH rat to limit urine flow was due either to pressor effects of ADH in increasing perfusion pressure or to a decreased responsiveness of the renal tubule to ADH. OHIO J. SCI. 80(2): 77, 1980 Since the development of the spontaneously hypertensive (SH) rat (Okamoto and Aoki 1963) through the inbreeding of Wistar rats showing elevated blood pressure, this strain has been extensively utilized as an animal model for the study of the pathophysiology of essential hypertension in humans. The majority of research has centered on the cardiovascular system, and it has been shown that the increase in blood pressure results from a generalized increase in total peripheral resistance (Folkow el al 1970) while cardiac output (Iriuchyima 1973) and regional blood flow (Nishiyama et al 1976) are unchanged from control values. Reports of abnormal contractile responses in SH rat visceral smooth muscle (Altman et al 1977) further suggest that hypertensive-normotensive differences may not be confined to the cardiovascular system. Morphological alterations of the kidney are not evident during the early stages of essential hypertension. On the other hand, a number of investigators have shown that humans with essential hypertension exhibit a more rapid sodium and water loss than normotensive controls Manuscript received 22 January 1979 and in revised form 10 July 1979 (#79-5). Present address: Dept. of Physiology, Marshall Univ., Huntington, WV. following volume expansion (Farnsworth 1946, Krakoff et al 1970). Observations on the excretion of acutely administered saline in SH rats are not as well denned but generally indicate an increased rate of sodium loss with oral loading (Willis et al 1976). Modifications of renal function in hypertensive individuals may also occur in the response to dehydration and in calcium excretion. Khokhar and Slater (1976) observed increased ADH excretion in hypertensive patients and a tenfold increase in plasma vasopressin levels has been observed in experimental hypertension (Mohring et al 1977). In regard to the renal handling of calcium, Miss-Pages and Gairard (1975) reported a significant correlation between calcium excretion and arterial pressure at the onset of hypertension of deoxycorticosterone acetate treated rats. To our knowledge, information is lacking concerning renal handling of calcium in the SH rat. In an effort to elucidate changes in renal function that may occur in hypertensive animals, urinary calcium excretion following oral loading was examined in SH rats. In addition, the urine concentrating ability during dehydration and the response to ADH challenge were examined in SH rats. METHODS Male spontaneously hypertensive (SH) rats of the strain developed by Okamoto and Aoki