Enhanced Na⁺-Ca²⁺ exchange activity in the regulation of vascular tone in spontaneously hypertensive rats

The role of sodium-calcium (Na⁺-Ca²⁺) exchange systems in nerve terminal and sarcolemmal membrane in the development of tension in aortic rings was compared between spontaneously hypertensive rats (SHR) and Wistar- Kyoto (WKY) rats. The tonic tension created by Na⁺-free solution was used to represent the Na⁺-Ca²⁺ exchange activity. Results showed that the tonic tension was significantly greater in SHR than in WKY rats. Dichlorobenzamil, a Na⁺-Ca²⁺ exchange activity inhibitor, markedly diminished the contraction in response to Na⁺-free solution in both rat strains. In the presence of prazosin, the tension induced by Na⁺-free solution was partly inhibited and this inhibitory effect was also greater in SHR than in WKY rats. Moreover, in aortic rings pretreated with prazosin and verapamil, the percentage of prazosin-and verapamil-insensitive maximum tension induced by Na⁺-free solution was also greater in the SHR than in WKY rats. Furthermore, the tonic tension was partly inhibited by in vitro adrenergic denervation of the aorta with 6-hydroxydopamine (6-OHDA). The inhibitory effect of 6-OHDA was also greater in the SHR than in WKY rats. The results suggest that the Na⁺-Ca²⁺ exchange activity in both the nerve terminal and vascular smooth muscle is increased in the SHR. The increase of Na⁺-Ca²⁺ exchange activity of the nerve terminal may partly participate in the regulation of vascular tone in the SHR.