Ca2+ mobilization by caffeine in single smooth muscle cells of the rat tail artery

Norepinephrine-induced calcium signaling pathways in afferent arterioles of genetically hypertensive rats

This study provides new information about the relative importance of calcium mobilization and entry in the renal vascular response to adrenoceptor activation in afferent arterioles isolated from 7- to 8-wk-old Wistar-Kyoto (WKY) and spontaneously hypertensive rats (SHR). Intracellular free calcium concentration ([Ca(2+)](i)) was measured in microdissected arterioles utilizing ratiometric photometry of fura 2 fluorescence. There was no significant strain difference in baseline [Ca(2+)](i). Norepinephrine (NE; 10(-6) and 10(-7) M) elicited immediate, sustained increases in [Ca(2+)](i). The general temporal pattern of response to 10(-6) M NE consisted of an initial peak and a maintained plateau phase. The response to NE was partially blocked by nifedipine (10(-6) M) or 8-(N,N-diethylamino) octyl-3,4,5-trimetoxybenzoate (TMB-8; 10(-5) M). A calcium-free external solution abolished the sustained [Ca(2+)](i) plateau response to NE, with less influence on the peak response. In the absence of calcium entry, TMB-8 (10(-5) M) completely blocked the calcium response to NE in WKY but not SHR, suggesting strain differences in mobilization. A higher concentration of TMB-8 (10(-4) M), however, blocked all discernible mobilization in both strains. We conclude that there are differences in Ca(2+) handling in renal resistance vessels between young WKY and SHR with respect to mobilization stimulated by alpha-adrenoceptors. Afferent arterioles of young SHR appear to have a larger inositol-1,4,5-trisphosphate-sensitive pool or release from a site less accessible to TMB-8.

Ionic mechanisms underlying spontaneous muscle contractions in the liver fluke, Fasciola hepatica

Spontaneous contractions of liver fluke muscle were abolished in Ca(2+)-free saline and by 100 microM nifedipine and reduced by 5 mM cadmium chloride, suggesting that they are dependent on extracellular Ca(2+). Caffeine (5 mM) significantly increased contraction amplitude and frequency. Ryanodine (100 microM) failed to block the caffeine response but significantly reduced spontaneous contraction frequency, suggesting that intracellular stores have a functional role. Cyclopiazonic acid (5 microM) had no effect on the caffeine response or spontaneous activity. 3-Isobutyl-1-methylxanthine (IBMX), forskolin, and 8-bromoadenosine 3',5'-cyclic monophosphate significantly increased spontaneous contractions, which implies that cAMP has a regulatory function in motility. Caffeine, however, produced no measurable increase in cAMP. The caffeine effect was inhibited by cadmium chloride and nifedipine, whereas IBMX-induced increases in amplitude were reduced by cadmium chloride. Thus caffeine and cAMP appear capable of opening plasma membrane Ca(2+) channels, but the involvement of cAMP in caffeine responses has not been proved.

Effects of paxilline on K+ channels in rat mesenteric arterial cells

The effects of paxilline, a mycotoxin, on whole-cell outward currents from freshly isolated cells of the rat mesenteric artery were studied. Paxilline inhibited a component of the outward current that was also sensitive to iberiotoxin. Inhibition could be observed at a concentration of 10 nM and complete inhibition of the iberiotoxin-sensitive current was achieved at 300 nM. The inhibition could be described by a single site of interaction with a Ki of 35.7 nM. Paxilline had no effect on the component of the current that was sensitive to 4-aminopyridine. It is concluded that paxilline is a potent inhibitor of large conductance Ca2+ -activated K+ currents in vascular smooth muscle cells.

A method for isolating adult and neonatal airway smooth muscle cells and measuring shortening velocity

Methods are described for isolating smooth muscle cells from the tracheae of adult and neonatal sheep and measuring the single-cell shortening velocity. Isolated cells were elongated, Ca2+ tolerant, and contracted rapidly and substantially when exposed to cholinergic agonists, KCl, serotonin, or caffeine. Adult cells were longer and wider than preterm cells. Mean cell length in 1.6 mM CaCl2 was 194 +/- 57 (SD) microm (n = 66) for adult cells and 93 +/- 32 microm (n = 20) for preterm cells (P < 0.05). Mean cell width at the widest point of the adult cells was 8.2 +/- 1.8 microm (n = 66) and 5.2 +/- 1.5 microm (n = 20) for preterm cells (P < 0.05). Cells were loaded into a perfusion dish maintained at 35 degreesC and exposed to agonists, and contractions were videotaped. Cell lengths were measured from 30 video frames and plotted as a function of time. Nonlinear fitting of cell length to an exponential model gave shortening velocities faster than most of those reported for airway smooth muscle tissues. For a sample of 10 adult and 10 preterm cells stimulated with 100 microM carbachol, mean (+/- SD) shortening velocity of the preterm cells was not different from that of the adult cells (0.64 +/- 0.30 vs. 0.54 +/- 0.27 s-1, respectively), but preterm cells shortened more than adult cells (68 +/- 12 vs. 55 +/- 11% of starting length, respectively; P < 0.05). The preparative and analytic methods described here are widely applicable to other smooth muscles and will allow contraction to be studied quantitatively at the single-cell level.

Caffeine- and noradrenaline-induced contractions of human vas deferens: contrasting effects of procaine, ryanodine and W-7

1. The effects of ryanodine, procaine, and N-(6-aminohexyl)-5-chloro-1-naphthalenesulfonamide (W-7) on noradrenaline (NA)- and caffeine-induced contractions of human vas deferens were investigated. 2. In the presence of nifedipine (1 microM), NA ( 100 microM) evoked biphasic contractions. Caffeine (20 mM) evoked repeatable tonic contractions. 3. Ryanodine (30 microM) inhibited the initial but not the secondary component of NA contractions. Procaine (1 and 10 mM) inhibited both components. Contractions induced by caffeine were unaffected by ryanodine or procaine. 4. The calmodulin antagonist W-7 (100 microM) reduced, in a reversible manner, both components of NA-induced response. Caffeine-induced contractions were also reduced in most preparations (8 of 11). In all preparations, contractions induced by caffeine were markedly inhibited after the washout of W-7. Higher doses of W-7 (300 microM) induced an increase in basal tension. 5. These results indicate that NA contracts the longitudinal muscle of human vas deferens by a ryanodine-sensitive calcium-induced calcium release (CICR) mechanism and, in addition, a ryanodine-insensitive pathway: both are sensitive to procaine. In contrast, contraction induced by caffeine is mediated by a pathway that is atypically insensitive to either ryanodine or procaine. The sensitivity of NA- and caffeine-induced contraction to W-7 suggests a role for calcium and its interaction with calmodulin in the response to both agents. The paradoxical action of W-7 is discussed.

Ryanodine- and cyclopiazonic acid-sensitive components in human vas deferens contractions to noradrenaline

1. The role of calcium stores in noradrenaline- (NA) and caffeine-induced contractions of human vas deferens were investigated using ryanodine and cyclopiazonic acid (CPA) in the presence of the calcium antagonist, nifedipine (1 microM) or in calcium-free/EGTA (1 mM) medium. 2. In either media, NA (100 microM) evoked biphasic contractions of longitudinal muscle and tonic circular muscle contractions. Caffeine (20 mM) evoked longitudinal but not circular muscle contractions. 3. Ryanodine (1-30 microM) or CPA (1-30 microM) inhibited contractions of circular muscle, and the initial but not secondary component of longitudinal muscle contraction to NA. 4. In the presence of nifedipine, pre-exposure to caffeine caused a potentiation of circular muscle, and the initial but not secondary longitudinal muscle contractions to NA. The presence of ryanodine or CPA during the caffeine pre-exposures effectively blocked the potentiation of the initial component and reduced the secondary component of the subsequent responses to NA in longitudinal muscle. 5. In calcium-free media, caffeine pre-exposures had little effect on subsequent NA-induced contractions in circular muscle, but reduced both components in longitudinal muscle. The presence of ryanodine or CPA during caffeine pre-exposures produced no further effects on either component of the subsequent NA-induced contraction in longitudinal muscle. 6 In the presence of nifedipine or in calcium-free media, repeated applications of caffeine evoked contractions in longitudinal muscle which were not blocked by either ryanodine or CPA. 7. These results suggest that circular muscle contraction by NA and the initial component of longitudinal muscle to NA both utilize an intracellular pool of calcium that is triggered via a ryanodine-sensitive mechanism and replenished via a CPA-sensitive Ca2+-ATPase. 8. In longitudinal muscle, both the secondary component of its response to NA and contraction to caffeine appear to involve an unusual but pharmacologically distinct (ryanodine- and CPA-insensitive) pathway. 9. The quiescence to caffeine of circular muscle may be caused by a relative absence of the ryanodine- and CPA-insensitive pathway.

Sources of calcium and alpha 1-adrenoceptor-mediated contraction in rat tail artery

1. The relative importance of intracellular and extracellular Ca2+ on alpha 1-adrenoceptor-mediated contraction by noradrenaline and St-587 has been studied and correlated with the binding characteristics in intact tail artery from Sprague-Dawley rats. 2. Noradrenaline and St-587 behaved as full agonists inducing a concentration-dependent vasoconstriction. 3. Nifedipine (1 microM and 10 microM) blocked by 50% (P < 0.001) and 75% (P < 0.001) respectively, the maximum contraction (Emax) induced by St-587. Nevertheless, to reach 40% inhibition of Emax on noradrenaline responses (P < 0.01), 10 microM nifedipine was necessary. 4. Both agonists induced a concentration-dependent accumulation of inositol phosphates. Noradrenaline behaved as a full agonist and St-587 as a partial agonist for this response. 5. [3H]-prazosin binding to intact tail artery rings was saturable and of high affinity (KD = 4.44 +/- 0.46 nM; Bmax = 36.35 +/- 4.22 fmol mg-1 tissue). 6. Competition curves for inhibition of specific [3H]-prazosin binding by WB-4101 suggest that the rat tail artery contains two alpha 1-adrenoceptor subtypes in an approximate ratio of 60:40. 7. After irreversible alkylation of alpha 1B-adrenoceptors with 100 microM chloroethylclonidine (CEC), nifedipine (1 microM) influenced to a greater extent the St-587- than the noradrenaline-induced contraction. 8. Our results indicate that the degree of participation of intracellular and extracellular Ca2+ sources, on the alpha 1-adrenoceptor-mediated contraction, depends on the agonist used. The two alpha 1-adrenoceptor subtypes observed in binding experiments seem to be unrelated to the Ca2+ sources used for contraction.

Effects of cyclopiazonic acid and thapsigargin on electromechanical activities and intracellular Ca2+ in smooth muscle of carotid artery of hypertensive rats

1. The effects of cyclopiazonic acid (CPA) and thapsigargin (TG), both of which are known to inhibit sarcoplasmic reticular Ca(2+)-ATPase, on the mechanical activities, intracellular Ca2+ level and electrical activities of smooth muscle of the carotid artery of stroke-prone spontaneously hypertensive rats (SHRSP) and Wistar Kyoto rats (WKY) were compared. 2. Both CPA and TG induced elevation of tension of the smooth muscle, which was composed of a phasic and a tonic component. The level of tension attained, especially the tonic component, was greater in the preparation from SHRSP. 3. The elevation of tension was associated with an increased intracellular Ca2+ level. Both the elevation of tension and the increase in intracellular Ca2+ were diminished by the removal of extracellular Ca2+ or by the application of verapamil. 4. The resting membrane potential of the preparations from SHRSP were depolarized to a greater extent than those from WKY.CPA depolarized the smooth muscle from both SHRSP and WKY, and the final level was also more depolarized in the preparation from SHRSP. 5. These results indicate that the elevation of tension induced by these drugs is mainly due to increased Ca2+ influx through voltage-dependent Ca2+ channels, and the difference in the action between the preparation from SHRSP and that from WKY can be explained mainly by the changes in the channels. 6. Thus, differences in the action of these drugs on the tension of smooth muscle between preparations from WKY and SHRSP can mainly be explained by the difference in the membrane potential which is related to the difference in voltage-dependent Ca2+ influx.