Effects of hypertonic solutions on contraction of frog tonic muscle fibers

Cytoskeletal remodeling in differentiated vascular smooth muscle is actin isoform dependent and stimulus dependent

Dynamic remodeling of the actin cytoskeleton plays an essential role in the migration and proliferation of vascular smooth muscle cells. It has been suggested that actin remodeling may also play an important functional role in nonmigrating, nonproliferating differentiated vascular smooth muscle (dVSM). In the present study, we show that contractile agonists increase the net polymerization of actin in dVSM, as measured by the differential ultracentrifugation of vascular smooth muscle tissue and the costaining of single freshly dissociated cells with fluorescent probes specific for globular and filamentous actin. Furthermore, induced alterations of the actin polymerization state, as well as actin decoy peptides, inhibit contractility in a stimulus-dependent manner. Latrunculin pretreatment or actin decoy peptides significantly inhibit contractility induced by a phorbol ester or an alpha-agonist, but these procedures have no effect on contractions induced by KCl. Aorta dVSM expresses alpha-smooth muscle actin, beta-actin, nonmuscle gamma-actin, and smooth muscle gamma-actin. The incorporation of isoform-specific cell-permeant synthetic actin decoy peptides, as well as isoform-specific probing of cell fractions and two-dimensional gels, demonstrates that actin remodeling during alpha-agonist contractions involves the remodeling of primarily gamma-actin and, to a lesser extent, beta-actin. Taken together, these results show that net isoform- and agonist-dependent increases in actin polymerization regulate vascular contractility.

Electrophoretic and functional identification of two troponin C isoforms in toad skeletal muscle fibers

The differential sensitivity of frog twitch and slow-tonic fibers to Ca(2+) and Sr(2+) suggests that these two fiber types express different troponin C (TnC) isoforms. To date, only one TnC isoform from anurans (resembling the mammalian fast-twitch isoform) has been isolated and characterized. In this study, we examined the possibility that anuran striated muscle contains more than one TnC isoform. Toward this end, we determined the TnC isoform composition of 198 single fibers from the rectus abdominis of the cane toad (a mixed slow-tonic and twitch muscle) and of toad cardiac muscle using a method that enables the identification of TnC isoforms on the basis of the effect of Ca(2+) on their electrophoretic mobility. The fibers were typed according to their myosin heavy chain (MHC) isoform composition. The data indicate that striated muscle of the cane toad contains two TnC isoforms, one of which (TnC-t) is present in all fibers displaying only twitch MHC isoforms and the other of which (TnC-T/c) is present in fibers displaying the tonic MHC isoform and in cardiac muscle. For a subpopulation of 15 fibers, the TnC isoform composition was also compared with Ca(2+) and Sr(2+) activation characteristics. Fibers containing the TnC-T/c isoform were approximately 3-fold more sensitive to Ca(2+), approximately 40-fold more sensitive to Sr(2+), and responded to a approximately 4.6-fold broader range of [Ca(2+)] than did fibers containing the TnC-t isoform. The Ca(2+) activation properties of toad fibers containing the TnC-T/c isoform appear to be consistent with the previously reported physiological characteristics of amphibian slow-tonic muscle fibers.

MARCKS is a major PKC-dependent regulator of calmodulin targeting in smooth muscle

Calmodulin (CaM) is a ubiquitous transducer of intracellular Ca(2+) signals and plays a key role in the regulation of the function of all cells. The interaction of CaM with a specific target is determined not only by the Ca(2+)-dependent affinity of calmodulin but also by the proximity to that target in the cellular environment. Although a few reports of stimulus-dependent nuclear targeting of CaM have appeared, the mechanisms by which CaM is targeted to non-nuclear sites are less clear. Here, we investigate the hypothesis that MARCKS is a regulator of the spatial distribution of CaM within the cytoplasm of differentiated smooth-muscle cells. In overlay assays with portal-vein homogenates, CaM binds predominantly to the MARCKS-containing band. MARCKS is abundant in portal-vein smooth muscle ( approximately 16 microM) in comparison to total CaM ( approximately 40 microM). Confocal images indicate that calmodulin and MARCKS co-distribute in unstimulated freshly dissociated smooth-muscle cells and are co-targeted simultaneously to the cell interior upon depolarization. Protein-kinase-C (PKC) activation triggers a translocation of CaM that precedes that of MARCKS and causes multisite, sequential MARCKS phosphorylation. MARCKS immunoprecipitates with CaM in a stimulus-dependent manner. A synthetic MARCKS effector domain (ED) peptide labelled with a photoaffinity probe cross-links CaM in smooth-muscle tissue in a stimulus-dependent manner. Both cross-linking and immunoprecipitation increase with increased Ca(2+) concentration, but decrease with PKC activation. Introduction of a nonphosphorylatable MARCKS decoy peptide blocks the PKC-mediated targeting of CaM. These results indicate that MARCKS is a significant, PKC-releasable reservoir of CaM in differentiated smooth muscle and that it contributes to CaM signalling by modulating the intracellular distribution of CaM.

Physiologic increases in extracellular sodium salt enhance coronary vasoconstriction and Ca2+ entry

High dietary sodium salt has been suggested to increase the risk of coronary vasospasm and coronary artery disease. However, whether high-sodium salt directly affects the mechanisms of coronary artery contraction is unclear. This study investigated whether physiologic and supraphysiologic increases in extracellular concentrations of sodium chloride ([NaCl]e) enhance the Ca2+ handling mechanisms of coronary smooth muscle contraction. Isometric contraction and 45Ca2+ influx were measured in endothelium-denuded porcine coronary artery strips incubated in Krebs solution (2.5 mM Ca2+) containing increasing [NaCl]e (120, 121, 123, 126, 130, 140, and 150 mM). Increasing [NaCl]e for 30 min did not increase the resting coronary tone or 45Ca2+ influx. 5-Hydroxytryptamine (5-HT) caused concentration-dependent increases in contraction and 45Ca2+ influx. Preincubation of coronary strips in increasing [NaCl]e for 30 min did not change the median effective dose of 5-HT. However, the magnitude of the 5-HT contraction and 45Ca2+ influx was significantly increased at 121-126 mM [NaCl]e. Preincubation with 2,4-dichlorobenzamil (10-5 M), inhibitor of the Na+/Ca2+ exchanger, or KB-R7943 (10-5 M), selective inhibitor of the reverse mode of the Na+/Ca2+ exchanger, abolished the increases in 5-HT contraction and 45Ca2+ influx at 121-126 mM [NaCl]e. Preincubation in Krebs solution containing 120 mM NaCl plus 1-6 mM LiCl or N-methyl-d-glucamine did not increase 5-HT contraction or 45Ca2+ influx. Higher [NaCl]e (140-150 mM) increased 5-HT-induced 45Ca2+ influx but inhibited 5-HT contraction. 5-HT (10-5 M)- and caffeine (25 mM)-induced contraction in Ca2+-free (2 mM EGTA) solution, a measure of Ca2+ release from the intracellular stores, was not affected by small increases in [NaCl]e (121-126 mM) but was inhibited at higher [NaCl]e (130-150 mM). Thus increases in [NaCl]e within the physiologic range enhance coronary smooth muscle contraction to 5-HT by a mechanism possibly involving Ca2+ entry via the reverse mode of the Na+/Ca2+ exchanger, but not Ca2+ release from the intracellular stores. The reduction of coronary contraction with supraphysiologic [NaCl]e in both Ca2+-containing and Ca2+-free Krebs could be related to excessive increases in ionic strength and may mask significant coronary vasoconstrictor effects of physiologic increases in [NaCl]e.

Gender differences in vascular smooth muscle reactivity to increases in extracellular sodium salt

Hypertension is more common in men and postmenopausal women than in premenopausal women, and gender differences in sensitivity to high dietary Na(+) salt have been suggested; however, the vascular mechanisms involved are unclear. We investigated whether increases in the extracellular concentration of Na(+) ([Na(+)](e)) enhance the mechanisms of vascular smooth muscle contraction and whether the vascular effects of [Na(+)](e) exhibit gender differences. Isometric contraction and (45)Ca(2+) influx were measured in endothelium-denuded aortic strips that were isolated from intact male, intact female, castrated male, and ovariectomized (OVX) female Sprague-Dawley rats and incubated in Krebs' solution (2.5 mmol/L Ca(2+)) containing increasing [Na(+)](e) by the addition of 1, 3, 6, 10, 20, and 30 mmol/L NaCl. Increasing [Na(+)](e) for 30 minutes did not increase the resting tone or (45)Ca(2+) influx in any group of rats. Phenylephrine (Phe) caused concentration-dependent increases in contraction and (45)Ca(2+) influx. In vascular strips from intact males, increasing [Na(+)](e) by the addition of 1 to 6 mmol/L NaCl significantly increased the magnitude of Phe contraction and (45)Ca(2+) influx. Further increases in [Na(+)](e) by the addition of 10, 20, and 30 mmol/L NaCl increased Phe-induced (45)Ca(2+) influx but inhibited Phe contraction, possibly because of excessive increases in ionic strength. Preincubation with 2,4-dichlorobenzamil (10(-5) mol/L), inhibitor of the Na(+)-Ca(2+) exchanger, or KB-R7943 (10(-5) mol/L), selective inhibitor of the reverse mode of the Na(+)-Ca(2+) exchanger, abolished the increases in Phe contraction and (45)Ca(2+) influx at increasing [Na(+)](e) obtained by the addition of 1 to 6 mmol/L NaCl. Preincubation in Krebs' solution containing control [Na(+)](e) plus 1 to 6 mmol/L LiCl or N-methyl-D-glucamine did not increase Phe contraction. In intact females, the Phe contraction and Ca(2+) influx were less than those in intact males and were not enhanced with increases in [Na(+)](e). The enhancement of Phe contraction and Ca(2+) influx with increases in [Na(+)](e) were not significantly different between castrated male rats and intact male rats but were greater in OVX female rats than intact female rats. In OVX female rats or castrated male rats treated with 17beta-estradiol (but not 17alpha-estradiol) subcutaneous implants, no significant changes in Phe contraction or Ca(2+) influx with increases in [Na(+)](e) were observed. In OVX female or castrated male rats simultaneously treated with 17beta-estradiol plus the estrogen receptor antagonist ICI 182,780, the Phe contraction and Ca(2+) influx were enhanced with increases in [Na(+)](e). Thus, in intact male rats, small physiological increases in [Na(+)](e) enhance smooth muscle contraction to Phe by a mechanism involving Ca(2+) entry, possibly via the reverse mode of the Na(+)-Ca(2+) exchanger. This mechanism appears to be reduced in the presence of endogenous or exogenous estrogen and thereby protects female rats against excessive increases in vascular reactivity during high dietary Na(+) intake.

Phosphotyrosine-dependent targeting of mitogen-activated protein kinase in differentiated contractile vascular cells

Tyrosine phosphorylation has been linked to plasmalemmal targeting of src homology-2-containing proteins, activation of mitogen-activated protein (MAP) kinase, nuclear signaling, and proliferation of cultured cells. Significant tyrosine phosphorylation and MAP kinase activities have also been reported in differentiated cells, but the signaling role of tyrosine-phosphorylated MAP kinase in these cells is unclear. The spatial and temporal relation between phosphotyrosine and MAP kinase immunoreactivity was quantified in differentiated contractile vascular smooth muscle cells by using digital imaging microscopy. An initial association of MAP kinase with the plasmalemma required upstream protein kinase C activity but occurred in a tyrosine phosphorylation-independent manner. Subsequent to membrane association, a delayed redistribution of MAP kinase, colocalizing with the actin-binding protein caldesmon, occurred in a tyrosine phosphorylation-dependent manner. The apparent association of MAP kinase with the contractile proteins coincided with contractile activation. Thus, tyrosine phosphorylation appears to target MAP kinase to cytoskeletal proteins in contractile vascular cells. This targeting mechanism may determine the specific destination and thereby the specialized function of MAP kinase in other phenotypes.