We compared the Ca2+ buffering function of the superficial sarcoplasmic reticulum (SR) during rest and during contraction in endothelium-denuded strips of small mesenteric arteries from 13-week-old spontaneously hypertensive rats (SHR) and normotensive Wistar-Kyoto rats (WKY). The addition of caffeine (1-20 mM) caused a transient contraction in both strains, and the contraction was significantly larger in SHR. When the SR Ca2+ buffering function was eliminated by cyclopiazonic acid (CPA; 10 microM) or thapsigargin (100 nM), both of which inhibit SR Ca2+-ATPase, or by ryanodine (10 microM), which depletes the SR Ca2+, there was a larger contraction in SHR than in WKY, suggesting that the Ca2+ buffering function of the SR during rest is more important in SHR than in WKY. Judging from the augmenting effects of these three agents on the contractile responses to Bay k 8644 (1-300 nM), an agonist of L-type Ca2+ channels, or norepinephrine (10(-9)-10(-4) M), an alpha-adrenoceptor agonist, the effects were significantly greater in SHR than in WKY. We conclude that 1) the Ca2+ influx during rest and during stimulation with Bay k 8644 or norepinephrine is strongly buffered by Ca2+ uptake into the superficial SR in the small mesenteric arteries from SHR and WKY; and 2) these Ca2+ buffering functions are increased in SHR because of the larger capacity of SR for Ca2+ storage.