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1. These studies describe the functional effects of modulation of the sarcoplasmic reticulum (SR) Ca2+ stores at three levels of the vasculature: (i) large arteries (rat and guinea-pig aorta); (ii) small resistance arteries (rat tail artery, rabbit mesenteric artery, dog mesenteric artery); and (iii) arterioles (guinea-pig submucosal arterioles of the small intestine). 2. All tissues responded to phenylephrine (PE; 10 mumol/L) with a transient contraction in Ca(2+)-free Krebs', reflecting Ca2+ release from PE-sensitive Ca2+ stores. After pretreatment with cyclopiazonic acid (CPA; 30 mumol/L) or thapsigargin (TSG; 1 mumol/L), putative SR Ca2+ pump inhibitors, the PE-induced contraction in a Ca(2+)-free medium was significantly inhibited in arterial tissues at all levels of the vasculature. Similarly, ryanodine (RYA; 30 mumol/L), an agonist that enhances Ca2+ release from the SR, also reduced the PE contraction in a Ca(2+)-free solution. 3. CPA or TSG alone in the presence of extracellular Ca2+, caused marked and sustained contraction in the rat and guinea-pig aorta and marked but transient or no contraction in the resistance arteries. In the rat and guinea-pig aorta, RYA caused a slowly developing tension. Little increase in basal tension was produced by RYA in resistance arteries and arterioles. 4. The findings show that an agonist-releasable Ca2+ pool is present at all levels of the vasculature that is independent of the size of the vessels and suggest that under normal physiological conditions there is an intimate balance between the roles of the plasma membrane and of the SR in the maintenance of vascular contractility. It appears that the role of the SR diminishes as the arteries become smaller, while Ca2+ fluxes across the plasma membrane predominates.

Application of a protein kinase C inhibitor, H7 (10, 30 and 100 JAM) or calphostin C (1 JiM), reduced the high K+ contraction but not that caused by an equi-osmolar concentration of mannitol. 6 The data suggest that hypertonic K+-induced contraction differs from that caused by hypertonicity or depolarization per se and invokes membrane enzyme activation.

1 Recently, 4-chloro-3-ethyl phenol (CEP) has been shown to cause the release of internally stored Ca 2+ , apparently through ryanodine-sensitive Ca 2+ channels, in fractionated skeletal muscle terminal cisternae and in a variety of non-excitable cell types. Its action on smooth muscle is unknown. In this study, we characterized the actions of CEP on vascular contraction in endothelium-denuded dog mesenteric artery. We also determined its ability to release Ca 2+ , by use of Ca 2+ imaging techniques, on dog isolated mesenteric artery smooth muscle cells and on bovine cultured pulmonary artery endothelial cells. 2 After phenylephrine-(PE, 10 mM) sensitive Ca 2+ stores were depleted by maximal PE stimulation in Ca 2+ -free medium, the action of CEP on re®lling of the emptied PE stores was tested, by ®rst preincubating the endothelium-denuded artery in CEP for 15 min before Ca 2+ was restored for a 30 min re®lling period. At the end of this period, Ca 2+ and CEP were removed, and the arterial ring was tested again with PE to assess the degree of re®lling of the internal Ca 2+ store. 3 In a concentration-dependent manner (30, 100 and 300 mM), CEP signi®cantly reduced the size of the post-re®lling PE contraction (49.4, 28.9 and 5.7% of control, respectively) in Ca 2+ -free media. This suggests that Ca 2+ levels are reduced in the internal stores by CEP treatment. CEP alone did not cause any contraction either in Ca 2+ -containing or Ca 2+ -free Krebs solution. 4 Restoring Ca 2+ in the presence of PE caused a large contraction, which re¯ects PE-induced in¯ux of extracellular Ca 2+ . The contraction of tissues pretreated with 300 mM CEP was signi®cantly less compared with controls. However, tissues pretreated with 30 and 100 mM CEP were una ected. Washout of CEP over 30 min produced complete recovery of responses to PE in Ca 2+ -free and Ca 2+ -containing medium suggesting a rapid reversal of CEP e ects. 5 Concentration-response curves were constructed for PE, 5-hydroxytryptamine (5-HT) and K + in the absence of and after 30 min pre-incubation with 30, 100 and 300 mM CEP. In all cases, CEP caused a concentration-dependent depression of the maximum response to PE (84.8, 43.4 and 11.6% of control), 5-HT (65.4, 25.7 and 6.9% of control) and K + (77.6, 41.1 and 10.8% of control). 6 Some arterial rings were pre-incubated with ryanodine (30 mM) for 30 min before the construction of PE concentration-response curves. In Ca 2+ -free Krebs solution, ryanodine alone did not cause any contraction. However, 58% (11 out of 19) of the tissues tested with ryanodine developed contraction (6.9+1.2% of 100 mM K + contraction, n=11) in the presence of external Ca 2+ . EC 50 values for PE in ryanodine-treated tissues (1.7+0.25 mM, n=16) were not signi®cantly di erent from controls (2.5+0.41 mM, n=22). Maximum contractions to PE (118.5+4.4% of 100 mM K + contraction, n=16) were also una ected by ryanodine when compared to controls (129+4.2%, n=23). 7 When fura-2 loaded smooth muscle cells (n=13) and endothelial cells (n=27) were imaged for ...