Potent vasoconstrictor actions of cyclopiazonic acid and thapsigargin on femoral arteries from spontaneously hypertensive rats

Abstract

1. The Ca²⁺ buffering function of sarcoplasmic reticulum (SR) in the resting state of arteries from spontaneously hypertensive rats (SHR) was examined. Differences in the effects of cyclopiazonic acid (CPA) and thapsigargin, agents which inhibit the Ca²⁺-ATPase of SR, on tension and cellular Ca²⁺ level were assessed in endothelium-denuded strips of femoral arteries from 13-week-old SHR and normotensive Wistar-Kyoto rats (WKY).

2. In resting strips preloaded with fura-PE3, the addition of CPA (10 μM) or thapsigargin (100 nM) caused an elevation of cytosolic Ca²⁺ level ([Ca²⁺]_i) and a contraction. These responses were significantly greater in SHR than in WKY.

3. The addition of verapamil (3 μM) to the resting strips caused a decrease in resting [Ca²⁺]_i, which was significantly greater in SHR than in WKY. In SHR, but not in WKY, this decrease was accompanied by a relaxation from the resting tone, suggesting the maintenance of myogenic tone in the SHR artery.

4. Verapamil (3 μM) abolished differences between SHR and WKY. The effects of verapamil were much greater on the contraction than on the [Ca²⁺]_i.

5. The resting Ca²⁺ influx in arteries measured after a 5 min incubation of the artery with ⁴⁵Ca was not increased by CPA or thapsigargin in either SHR or WKY. The net Ca²⁺ entry measured after a 30 min incubation of the artery with ⁴⁵Ca was decreased by CPA or thapsigargin in both SHR and WKY. The resting Ca²⁺ influx was significantly higher in SHR than in WKY, and was decreased by nifedipine (100 nM) in the SHR artery, but was unchanged in the WKY artery.

6. The resting ⁴⁵Ca efflux from the artery was increased during the addition of CPA (10 μM). This increase was less in SHR than in WKY. The resting ⁴⁵Ca efflux was the same in SHR and WKY.

7. These results suggest that (1) the Ca²⁺ influx via L-type voltage-dependent Ca²⁺ channels (VDCCs) was increased in the resting state of the SHR femoral artery, (2) the greater part of the increased Ca²⁺ influx was buffered by Ca²⁺ uptake into the SR and some Ca²⁺ reached the myofilaments resulting in the maintenance of the myogenic tone, and (3) therefore the functional elimination of SR by CPA or thapsigargin caused a large elevation of [Ca²⁺]_i and a potent contraction in this artery. During this process, the contraction was mainly due to the basal Ca²⁺ influx via L-type VDCCs. The present study also showed the existence of a relatively large compartment of [Ca²⁺]_i which does not contribute to the contraction during the addition of CPA or thapsigargin.

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