Abstract P103: Cellular Stretch Induces Higher Intracellular Calcium Increases in Dahl Salt-Sensitive Than Salt-Resistant Rat Thick Ascending Limbs: Role of Transient Receptor Potential Vanilloid 4

Abstract

The thick ascending limb (TAL) reabsorbs 30 % of the filtered NaCl playing a pivotal role in the regulation of salt homeostasis. Abnormal reabsorption of NaCl in this segment causes salt-sensitive hypertension. We showed that flow-induced cellular stretch stimulates superoxide production in epithelial cells from TALs. Dahl salt-sensitive rat (DSS) TALs produce more superoxide than those from salt-resistant rats (DSR). Mechanical stimulation exerted by cellular stretch increases intracellular calcium (Cai) in several types of cells. We have shown that luminal flow-induced increases in Cai is mediated by Transient Receptor Potential Vanilloid (TRPV4). Therefore, we hypothesized that cellular stretch increases Cai in TALs from DSS rats more than those from DSR rats due to elevated TRPV4 activity. We measured Cai using the calcium-sensitive dye Fura-2 in isolated, perfused DSR and DSS TALs. Cellular stretch increased Cai by 243 ± 51 nM in DSS TALs (n=9; p<0.0008 vs. no stretch conditions) and by 124 ± 27 nM in DSR TALs (n=10; p<0.0005 vs no stretch conditions). Cellular stretch-induced Cai increases were significantly higher in the DSS group (p<0.05 vs DSR). When these animals were fed a high-salt diet (4% NaCl) similar responses were obtained. Cellular stretch increased Cai by 236 ± 58 nM in DSS TALs (n=7; p<0.004 vs no stretch conditions) and by 92 ± 15 nM in DSR TALs (n=6; p<0.0008 vs no stretch conditions). To study whether DSS and DSR TALs express different amounts of TRPV4 we performed Western blots. We found that TRPV4 protein expression was similar in TALs lysates from both strains fed either a control standard or a high-salt diet. We then tested the effects of transfecting DSS TALs with TRPV4-small hairpin (sh) RNA on stretch-induced Cai increases. Under these circumstances, the difference in stretch-induced Cai increases between DSS and DSR TALs was not significant (75 ± 15 nM in DSS vs 56 ± 28 nM vs DSR, n=4 for each group) . We conclude that DSS TALs are more sensitive to increases in cellular stretch than DSR TALs possibly due to enhanced TRPV4 activity.