Ontogeny of Calcium Transport By Intestinal Golgi in Spontaneously Hypertensive Rats and Genetically Matched WKY Rats

Abstract

ABSTRACT. Our studies were designed to characterize calcium transport by intestinal Golgi vesicles in spontaneously hypertensive rats (SHR) and their genetically matched control, Wistar-Kyoto rats (WKY). The biochemical purity of the intestinal Golgi in SHR and WKY was validated by marker enzyme studies. Calcium uptake by Golgi vesicles represented transport into the intravesicular space as evidenced by temperature dependency and by calcium ionophore A23187-induced calcium efflux experiments. ATP-driven calcium uptake was stimulated severalfold compared with uptake in the absence of ATP and adenylyl-(/3-7-rnethylendiphosphate) (nonhydrolyzable ATP) in both SHR and WKY. ATP-dependent calcium uptake was significantly higher in WKY compared with SHR at early times points, 15 s-5 min ( p < 0.05-0.01). The initial rate of calcium uptake was linear up to 60 s. Epidemiologic studies suggested an inverse relationship between blood pressure and dietary calcium intake (1-5). Oral calcium supplementation in humans resulted in reduction in blood pressure (6). Furthermore, low serum level of ionized calcium has been described in humans with essential hyperten- Supported by Grant no. NIH HL3796 1. sion (7). These studies suggest an alteration of calcium homeostasis in humans with essential hypertension.The SHR is the most widely used animal model of human essential hypertension (8). Calcium metabolism in SHR has been shown by several studies to be abnormal. These abnormalities include reduced serum ionized calcium, increased serum parathormone levels, hypercalciuria, and abnormal vitamin D metabolism (9-16). Studies on calcium transport using in vivo and in vitro techniques, in general, have shown decreased calcium uptake by the small intestine of the SHR when compared with WKY (12,15,16,17,18). Calcium transport across the enterocyte, however, represents three steps that include: entry across brush border membrane, cytosolic movement, and exit of the basolateral membrane. To our knowledge, there are no studies to address the cytosolic movement of calcium within the enterocytes. Because the Golgi apparatus plays a major role in intracellular calcium handling (19,20) and abnormalities in intracellular calcium handling have been reported in tissues of SHR (2 I), we postulated that calcium uptake by Golgi apparatus might be abnormal.Therefore, our studies were designed to characterize calcium transport by intestinal Golgi of the SHR and their genetically matched control, WKY. Furthermore, calcium uptake was studied before (3-wk-old rats) and after development of hypertensive state (1 2-wk-old). MATERIALS AND METHODS Animals and materials.Both SHR and WKY were obtained from Taconic Farms (Germantown, NY). Adult rats (12-14 wk old) and weanling rats (21 + 1 d old) were used to prepare the intestinal Golgi vesicles. Adult and weanling rats were fed a regular diet that contained 1.2% calcium, 0.8 phosphate, and 1700 U/ 100 g of ergocalciferol (Teklad Diets, Madison, WI). 45Ca (0.37-1.5 GBq/mg) was obtained from New England Nuclear C...