Ultrastructure, calcium accumulation, and contractile response in smooth muscle

Effects of methoxamine and barium on 45Ca2+ fluxes in the rat vas deferens

The aim of this study was to determine whether methoxamine and barium stimulate 45Ca2+ uptake or efflux in the rat vas deferens in a manner that correlates with their contractile activity, and whether 45Ca2+ movements are inhibited by verapamil or nifedipine. Basal La(3+)-resistant (cellular) 45Ca2+ uptake was significantly greater in the epididymal half (791 +/- 27 nmol g-1) than in the prostatic half (654 +/- 14 nmol g-1) of the rat vas deferens and was unaffected by verapamil (61 microM) or nifedipine (14 microM). Methoxamine (8 microM) was without effect on 45Ca2+ uptake in either half but BaCl2 (1 mM) increased 45Ca2+ uptake by 31% in the prostatic half and by 22% in the epididymal half. The barium-induced increases in 45Ca2+ uptake were markedly reduced or abolished by verapamil (2 microM) or nifedipine (0.3 microM), which at these concentrations have no effect on the rhythmic contractions but abolish the initial small phasic contraction induced by barium. The basal rate of 45Ca2+ efflux from the intact vas deferens (into Ca2+ containing Krebs-Henseleit solution or into Ca-free Krebs-Henseleit solution +/- EGTA 0.05 mM) was not affected by verapamil (61 microM) or nifedipine (14 microM). Methoxamine (8 microM) produced a marked, transient and reversible increase in 45Ca2+ efflux into 2.5 mM CaCl2 Krebs-Henseleit in 50% of the intact vasa deferentia examined which was augmented by verapamil (61 microM). BaCl2 (1 mM) produced a small increase in 45Ca2+ efflux into Ca(2+)-containing and Ca(2+)-free Krebs-Henseleit solutions from some intact vasa deferentia and this was not inhibited by nifedipine (14 microM).(ABSTRACT TRUNCATED AT 250 WORDS)

Localization of calcium in vas deferens using 45Ca EM autoradiography: relationship to species and the effect of 45Ca removal

The distribution of calcium was determined in the vas deferens of the guinea pig using 45Ca electron microscopic autoradiography of rapidly frozen, freeze-dried, and embedded tissue. A selective accumulation of calcium at the plasma membrane and SR was observed in vas deferens that had been incubated in 45Ca for 65-85 min prior to rapid freezing. Rinsing the tissue in nonradioactive calcium for 6 min prior to rapid freezing significantly altered the distribution of calcium among the plasma membrane, mitochondria, and cytoplasmic matrix. The influence of species on the observed distribution of calcium was also examined. The distribution of calcium in the guinea pig vas deferens was not significantly different from that in the rabbit vas deferens when the tissues were prepared under identical conditions.

The cellular distribution of calcium in freeze-dried rabbit vas deferens using EM autoradiography

The role of calcium in initiating smooth muscle contraction is widely accepted. The sources of this calcium are thought to be located both intracellularly and extracellularly. We have recently developed a method by which the cellular localization of calcium in smooth muscle can be determined. This method involves exposing the tissue to 45Ca, rapidly freezing and vacuum dehydrating the tissue, and preparing the tissue for electron microscopic autoradiography (EM ARG). In the present study the distribution of calcium in control and potassium-contracted tissue of the rabbit vas deferens was compared. No significant differences in distribution were observed in the two treatments. This finding provides morphological support for the hypothesis that the calcium used in potassium-induced contraction is primarily of extracellular origin. In addition, significant sequestration by intracellular organelles does not occur during a potassium contraction. In other experiments, the effect of rinsing tissue in cold calcium prior to freezing was investigated. From these data is appears that calcium is removed from the cytoplasmic matrix, plasma membrane, and organelles in a nonuniform manner. Further investigation into these findings should enable us to characterize more precisely the intracellular redistribution of calcium that occurs as a result of a variety of physiological manipulations.

Cellular calcium and the contraction induced by prostaglandin F2 alpha in feline cerebral arteries

The influences of different calcium-entry blockers, sialidase and caffeine on the biphasic contraction induced by prostaglandin (PG) F2 alpha in the feline basilar artery (BA) were studied in calcium-free medium. After incubation in calcium-free solution, PGF2 alpha induced a contraction of the BA amounting to 87% of the contraction in calcium-containing solution. The response was biphasic in 41 out of 42 vessel segments. PGF2 alpha-induced contractions were markedly attenuated in TRIS-buffered solutions as compared to contractions in Krebs solution. PGF2 alpha failed to induce a biphasic contraction (8 out of 9 preparations) in calcium-free HEPES-buffered solution. Calcium entry blockade with 1 mM manganese or 10(-5) M diltiazem abolished the second and major phase of the PGF2 alpha-induced contraction in calcium-free Krebs solution. The second contraction phase was also eliminated in four out of five preparations pretreated with sialidase (1 unit/ml for 30 min.), but was unaffected by a brief exposure to 20 mM caffeine in calcium-free medium. The present findings strongly support previous suggestions that a major part of the PGF2 alpha-induced contraction in calcium-free medium is mediated via the release of calcium bound to the exterior aspect of the cell membrane.

Characterization of the prostanoid receptors and of the contractile effects of prostaglandin F2 alpha in human pial arteries

The contractile and relaxant effects of various prostanoids were studied on isolated human pial arteries. Contractions were elicited with the following order of potency: U46619 approximately equal to U44069 greater than PGB2 greater than PGF2 alpha greater than PGE2 approximately equal to PGD2 approximately equal to PGF1 alpha greater than or equal to TXB2, indicating that prostanoid-induced contractions probably are mediated by a thromboxane-sensitive receptor. Relaxation of PGF2 alpha-contracted arteries was induced with the order of potency: PGE2 greater than PGE1 greater than PGD2 approximately equal to PGD1. Vessels contrated by K+ were relaxed only by PGE1. Since PGI2 was previously found to be more potent than all the prostanoids tested in the present study, relaxant responses are probably mediated via a PGI2-sensitive receptor. The role of free extracellular and cellularly bound calcium for the contractile effects of PGF2 alpha and K+ were estimated by incubating the arteries for various times in calcium-free medium containing 10(-5) M EGTA. Incubation for 5-10 min abolished K+-induced contractions, whereas after 40 min of incubation PGF2 alpha still induced contractions that reached 70% of control. The PGF2 alpha-induced contraction was biphasic in 8 out of 10 preparations. The second phase could be eliminated by increasing the EGTA-concentration to 10(-4) M, as well as by nifedipine pretreatment. In calcium-free, high K+ medium calcium-induced contractions were elicited at lower concentrations in the presence of PGF2 alpha. The results suggest that PGF2 alpha-induced contractions in human pial arteries are relatively independent of free extracellular calcium. PGF2 alpha may promote trans-membrane influx of calcium, as well as release calcium from seemingly superficially located cellular stores.

Effects of prostanoids on isolated feline cerebral arteries. II. Roles of extra- and intracellular calcium for the prostaglandin F2 alpha-induced contraction

The roles of extra- and intracellular calcium for the contractile effects of PGF2 alpha in the feline basilar artery (BA) were investigated. Comparisons were made with contractions induced by K+ and noradrenaline (NA). Addition of nifedipine to PGF2 alpha- or K+ (124 mM)-contracted arteries resulted in an incomplete relaxation, whereas NA-contracted vessels were completely relaxed. Incubation of the preparations in a calcium-free medium containing 10(-5) M EGTA for 5-10 min almost abolished contractions induced by K+ and NA. In contrast, 63% of the response to PGF2 alpha remained after pretreatment of the arteries in a calcium-free solution for 40 min; PGF2 alpha produced a biphasic contraction in 17 out of 20 preparations consisting of a rapidly developing initial phase followed by a second increase in tension after 1-6 min. The second phase was absent if the EGTA-concentration was increased to 10(-4) M, or if the arteries were pre-treated with nifedipine. After incubation of the arteries in a calcium-free medium for 40-120 min and K+-depolarization, re-addition of calcium elicited contractions at lower concentrations in the presence of PGF2 alpha than in controls. The results suggest that PGF2 alpha-induced contractions in the feline BA are considerably less dependent on extracellular calcium than contractions evoked by K+ or NA. PGF2 alpha appears to be able to release calcium from two cellular stores, and may also promote calcium influx through the cell membrane.

A 45Ca autoradiographic and stereological study of freeze-dried smooth muscle of the guinea pig vas deferens

In an effort to more clearly elucidate the role of cellular structures as calcium sinks and sources in smooth muscle cells, the intracellular distribution of radioactive calcium was evaluated by a new method based on freeze-drying. The guinea pig vas deferens was exposed to a physiological salt solution that contained 45Ca. The muscle was then freeze-dried and prepared for electron microscope autoradiography. The grain density over the plasma membrane, mitochondria, and sarcoplasmic reticulum (SR) was significantly greater than that of the matrix. These results suggest that the plasma membrane, mitochondria and SR have the capacity to accumulate calcium. Which of these structures serve as a source of calcium for contraction remains to be determined. A stereological comparison between freeze-dried and conventionally prepared smooth muscles revealed several differences. The cross-sectional area of freeze-dried cells was about twice that of conventionally prepared cells. Moreover, mitochondria and sub-surface vesicles occupied a significantly smaller percentage of the cell in the freeze-dried tissue than they did in the conventionally prepared tissue.

Molluscan visceral muscle fine structure. General structure and sarcolemmal organization in the smooth muscle of the intestinal wall of Buccinum undatum L

Fine structure of intestinal muscle in the gastropod Buccinum undatum is described. Myofibrillar organization is typical of non-pseudostriated molluscan muscles. The dense body system is poorly developed but there are extensive attachments plaques. The sarcolemma is elaborately modified. Deep infoldings of the membrane give the cells an irregular outline. Such infoldings enclose extracellular matrix and are associated with attachment plaques. Arising from these and from the general sarcolemma are numerous tubular membranous invaginations ending blindly at varying depth in the sarcoplasm. These structures have a helical coat of particles on the cytoplasmic face. Associated with both types of invagination are subsarcolemmal vesicles. The possibility that the tubular invaginations are analogues of vertebrate smooth muscle caveolae or striated muscle T-tubules and that the vesicles are the corresponding sarcoplasmic reticulum is discussed. The occurrence of such structures in molluscan muscle and elsewhere is reviewed.

Action of indapamide on excitation-contraction coupling in vascular smooth muscle

The effects of indapamide on electrophysiological and mechanical parameters of longitudinal smooth muscle strips isolated from mammalian portal vein were studied by means of a double sucrose gap method associated with a photoelectric device for recording contraction. Indapamide (10(-4) M) reduced both the amplitude of the action potential and the contraction. The calcium inward current decreased and consequently the phasic contraction was also reduced. The potassium outward current was diminished while the tonic contraction was not modified significantly. The depressant nature of the indapamide response could counterbalance the stimulating action of angiotensin II but not that of noradrenaline. The results suggest that indapamide acts primarily on the plasma membrane of vascular smooth muscle by reducing the transmembrane calcium current, although a secondary decrease in the intracellular bound calcium could not be completely excluded.