Upregulation of Na+/Ca2+ Exchanger and TRPC6 Contributes to Abnormal Ca2+ Homeostasis in Arterial Smooth Muscle Cells from Milan Hypertensive Rats

The Milan hypertensive strain (MHS) of rats is a model for hypertension in humans. Inherited defects in renal function have been well studied in MHS rats, but the mechanisms that underlie the elevated vascular resistance are unclear. Altered Ca(2+) signaling plays a key role in the vascular dysfunction associated with arterial hypertension. Here we compared Ca(2+) signaling in mesenteric artery smooth muscle cells from MHS rats and its normotensive counterpart (MNS). Systolic blood pressure was higher in MHS than in MNS rats (144 +/- 2 vs. 113 +/- 1 mmHg, P < 0.05). Resting cytosolic free Ca(2+) concentration (measured with fura-2) and ATP-induced Ca(2+) transients were augmented in freshly dissociated arterial myocytes from MHS rats. Ba(2+) entry activated by the diacylglycerol analog 1-oleoyl-2-acetyl-sn-glycerol (a measure of receptor-operated channel activity) was much greater in MHS than MNS arterial myocytes. This correlated with a threefold upregulation of transient receptor potential canonical 6 (TRPC6) protein. TRPC3, the other component of receptor-operated channels, was marginally, but not significantly, upregulated. The expression of TRPC1/5, components of store-operated channels, was not altered in MHS mesenteric artery smooth muscle. Immunoblots also revealed that the Na(+)/Ca(2+) exchanger-1 (NCX1) was greatly upregulated in MHS mesenteric artery (by approximately 13-fold), whereas the expression of plasma membrane Ca(2+)-ATPase was not altered. Ca(2+) entry via the reverse mode of NCX1 evoked by the removal of extracellular Na(+) induced a rapid increase in cytosolic free Ca(2+) concentration that was significantly larger in MHS arterial myocytes. The expression of alpha(1)/alpha(2) Na(+) pumps in MHS mesenteric arteries was not changed. Immunocytochemical observations showed that NCX1 and TRPC6 are clustered in plasma membrane microdomains adjacent to the underlying sarcoplasmic reticulum. In summary, MHS arteries exhibit upregulated TRPC6 and NCX1 and augmented Ca(2+) signaling. We suggest that the increased Ca(2+) signaling contributes to the enhanced vasoconstriction and elevated blood pressure in MHS rats.