Intestinal calcium transport in spontaneously hypertensive rats and their genetically matched Wistar-Kyoto rats I. Endoplasmic reticulum


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Abstract

The present study was designed to investigate calcium uptake by the intestinal endoplasmic reticulum in the spontaneously hypertensive rat (SHR) and its genetically matched normotensive control, the Wistar-Kyoto rat (WKY). The calcium uptake was characterized before (weanling) and after (adult) development of hypertension in SHR and their age-matched WKY controls. Adenosine 5'-triphosphate (ATP)-driven calcium uptake with time was stimulated several-fold compared with the no-ATP condition, and the maximal uptake occurred after 15min in both SHR and WKY. The maximal ATP-dependent calcium uptake was significantly greater in the intestinal endoplasmic reticulum of adult SHR than in that of WKY (P<0.05). The initial rate of calcium uptake was linear up to 30 s in both SHR and WKY. The kinetics of ATP-dependent calcium uptake at a concentration between 0.1 and 1.0µmol/l showed a maximal uptake (Vmax) of 1.89 ± 0.3 and 1.06 ± 0.1 nmol/mg protein per 15 s for adult SHR and WKY, respectively (P<0.02), and binding constant (Km) values of 0.15 ± 0.08 and 0.12 ± 0.05µmol/1, respectively. The ontogeny of calcium uptake by intestinal endoplasmic reticulum was characterized before the development of hypertension in weanling (3-week-old) SHR and WKY. In both SHR and WKY weanlings, calcium uptake by the intestinal endoplasmic reticulum was enhanced several-fold by the presence of ATP compared with the no-ATP condition, and the maximal uptake occurred after 15min. The maximal ATP-dependent calcium uptake was significantly greater in 3-week-old SHR than in age-matched WKY (P<0.05). Kinetic analysis of the ATP-dependent calcium uptake showed a Vmax of 0.8 ± 0.08 and 0.38 ± 0.06 nmol/mg protein per 15 s for weanling SHR and WKY, respectively (P<0.01), and Km values of 0.11 ± 0.04 and 0.18 ± 0.09µmol/l, respectively. These results suggest that SHR exhibit abnormalities in intracellular calcium handling and that these changes are present before the appearance of hypertension.

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