Calcium and sodium distribution and movements in smooth muscle

Urogenital alterations in aged male caveolin-1 knockout mice

PURPOSE

Caveolae are flask-shaped invaginations of the plasma membrane formed by the oligomerization of caveolins. Because only smooth muscle contains all caveolin (Cav) family members (Cav-1, 2 and 3), we examined the contribution of each caveolin to urogenital smooth muscle structure/function.

MATERIALS AND METHODS

WT, Cav-1, 2, 3 and -1/3 knockout (KO) mouse bladders were characterized by Western blot, co-immunoprecipitation, immunofluorescence microscopy, electron microscopy, histochemistry and pharmacological techniques. Cystometric analysis was performed in conscious, freely moving mice. Other urogenital organs were investigated by histological analysis.

RESULTS

The loss of bladder Cav-1 results in a marked decrease in Cav-2 but not Cav-3 expression. Ablation of Cav-3 fails to alter Cav-1 or Cav-2 expression. Deletion of Cav-1 results in the almost complete loss of caveolae, while Cav-2 KO and Cav-3 KO mouse smooth muscle showed a normal number of caveolae. The loss of Cav-1 generated caveolae led to significant urogenital changes in male mice (most marked by 12 months of age), namely 1) bladder weight-to-body weight ratios were increased, 2) the bladder smooth muscle layer was thickened, 3) the bladders had increased baseline, threshold and spontaneous pressures, 4) bladder strips showed a decreased contractile response to carbachol and KCl, and 5) these smooth muscle changes were accompanied by marked fluid accumulation in the prostate and seminal vesicles, with intracellular vacuolization in the kidneys. As such, male Cav-1 KO mice may be a useful animal model for studying LUTD (lower urinary tract dysfunction) that is so prevalent in aging male patients.

CONCLUSIONS

The loss of Cav-1 and, thus, of most smooth muscle cell caveolae results in significant bladder dysfunction and urogenital organ changes in aged male mice.

Utilization of electron probe microanalysis in gadolinium-treated mice

Gadolinium is used as a contrast media for magnetic resonance imaging and, experimentally, to block Kupffer cell phagocytosis. In this study, we utilize electron probe microanalysis to determine the subcellular localization of gadolinium chloride (GdCl3) administered to mice in a short-term toxicology study. Male CD-1 mice were administered 0.0, 2.5, or 8.0 mg/kg GdCl3 iv for 14 consecutive weekdays. Liver-associated enzymes were significantly elevated in high-dose animals only and correlated histologically with multifocal, hepatocellular degeneration associated with a neutrophilic infiltrate. Morphological investigations were performed on high-dose animals. Hepatocytes and Kupffer cells had morphologic features of cellular injury consisting of swollen mitochondria and vesiculated profiles of endoplasmic reticulum. Hepatocytes, Kupffer cells, bile canaliculi, and neutrophils in the liver contained discrete aggregates of electron-dense granular material, as did pulmonary interstitial macrophages, splenic macrophages, and mesangial cells of the renal glomerulus. The intracellular granular material in the liver, lung, spleen, and kidney was confirmed as gadolinium by qualitative electron probe microanalysis. These results document both hepatic and extra-hepatic accumulation of gadolinium in cells of the mononuclear phagocytic system and highlight the importance of electron probe microanalysis in toxicologic assessment.

Cytochemical localization of Ca(2+)-ATPases and demonstration of ATP-dependent calcium sequestration in giant smooth muscle fibres of Beroe

A cytochemical analysis of the mechanisms underlying cytosolic calcium regulation was undertaken in the giant smooth muscle fibres of the marine invertebrate Beroe. The ability of the sarcoplasmic reticulum to accumulate Ca2+ was demonstrated on living skinned single cells. In the presence of oxalate, and physiological concentrations of Ca2+, calcium oxalate crystals were formed in the lumen of tubules and cisternae of the sarcoplasmic reticulum. The subcellular distribution of Ca(2+)-ATPase was studied with a cytochemical technique; a dense precipitate resulting from Ca(2+)-ATPase activity was found on the plasma membrane, on the membranes of tubules and cisternae of the sarcoplasmic reticulum, and in mitochondria.

Modulatory role of magnesium on the contractile response of rat aorta to several agonists in normal and calcium-free medium

Acute withdrawal of external Mg2+ increased basal tone of rat isolated aorta incubated in the presence of Ca2+. Above normal levels of Mg2+ (1-4 mM) inhibited basal tone while much higher levels of the divalent cation (64-256 nM) evoked contractile responses regardless of the presence of Ca2+. Contractile responses to noradrenaline (1 microM) and KCl (80 mM) were inhibited by addition of cumulative concentrations of Mg2+. Acetylcholine-induced contractions in the presence of physiological concentrations of Mg2+ (1 mM) decreased gradually to the basal tone, but a sustained contraction was observed in the absence of this ion. In Ca(2+)-free medium, acetylcholine-induced phasic responses indicate the existence of an acetylcholine-sensitive Ca2+ store. KCl induced contraction only in Krebs solution, although a small residual contraction could be observed in Ca(2+)-free medium in some experiments. Mg(2+)-depletion in the extracellular medium increased contractile responses induced by acetylcholine and KCl in Ca(2+)-free medium. These results suggest that extracellular Mg2+ modulates basal tone, Ca2+ channels and responsiveness to various agents in the absence of Ca2+.

Na+-Ca2+ exchange in cultured vascular smooth muscle cells

Vascular smooth muscle cells (VSMC) contract as intracellular free calcium ([Ca2+]i) rises. While Na+-Ca2+ exchange has been proposed to contribute to transmembrane Ca2+ flux, its role in cultured VSMC is unknown. Accordingly, we have investigated the role of Na+-Ca2+ exchange in unidirectional and net transmembrane Ca2+ fluxes in cultured rat aortic VSMC under basal conditions and following agonist-mediated stimulation. Transmembrane Ca2+ uptake was significantly increased in response to a low external Na+ concentration ([Na+]o) compared with 140 mM [Na+]o. Na+-dependent Ca2+ uptake in response to low [Na+]o was further increased by intracellular Na+ loading by preincubation of the VSMC with 1 mM ouabain. Under steady-state conditions, Ca2+ content varied inversely with [Na+]o, increasing from 1.0 nmol Ca2+/mg protein at 140 mM [Na+]o to 4.0 nmol Ca2+/mg protein at 20 mM [Na+]o. Increasing [K+]o to 55 mM also enhanced Na+-dependent Ca2+ influx. Augmentation of Ca2+ uptake with K+ depolarization was not significantly inhibited by the calcium channel antagonist verapamil. Transmembrane Ca2+ efflux was increased in response to 130 mM [Na+]o compared with zero [Na+]o (iso-osmotic substitution with choline+), and was further stimulated by the vasoconstrictor angiotensin II, which is known to elevate [Ca2+]i. These changes in [Ca2+]i were studied directly using fura-2 fluorescence measurements. Elevated [Ca2+]i levels returned to baseline more rapidly in the presence of normal (130 mM) [Na+]o compared with zero [Na+]o (iso-osmotic substitution with choline+). These findings suggest that a bidirectional Na+-Ca2+ exchange mechanism is present in cultured rat aortic VSMC.(ABSTRACT TRUNCATED AT 250 WORDS)

Use of tension measurements to delineate the mode of action of vasodilators

Direct experimental procedures to delineate the modes of inhibition of drugs on vascular smooth muscle contractility are described. In isolated rabbit aortic strips, high concentrations of KCl and norepinephrine induce sustained contractions that depend upon Ca2+ influx mediated by selective activation of voltage-dependent and receptor-linked Ca2+ channels, respectively. In the absence of external Ca2+, norepinephrine also induces a transient contraction that is dependent upon release of cellular bound Ca2+. The contractile responses are differentially susceptible to vasodilators acting on different mechanisms. The Ca2+ channel blockers selectively inhibit KCl-induced contraction. The selective inhibitors of norepinephrine-induced contraction include nitro compounds, atrial natriuretic peptide, acetylcholine and other stimulants of endothelium-derived relaxing factor, beta-adrenergic agonists, forskolin, adenosine, and metabolic inhibitors. The nonselective inhibitors of these contractions include the inhibitors of contractile filaments, such as calmodulin inhibitors, and the inhibitors with multiple sites of action, such as papaverine. Although the inhibitors of Ca2+ release from storage site, such as ryanodine, may not inhibit these contractions, these inhibitors inhibit the norepinephrine-induced transient contraction in Ca2+-free solution. Thus, primary evaluation (screening) of drugs affecting vascular smooth muscle contraction can be performed by analyzing their effects on contractile responses of isolated rabbit aorta. Furthermore, methods to define more detailed sites of action of drugs are also described.

Ca2+ and K+ current in cultured vascular smooth muscle cells from rat aorta

Ca2+ (ICa) and K+ (IK) currents were recorded in single cultured cells from rat aorta using the whole cell clamp technique with patch electrodes. ICa was detected at -30 mV, and at 20 mV it reached a peak in about 10 ms and decayed with a t1/2 = 50 ms. The mean maximum slope conductance (GCa) was 30 microseconds/cm2. lK was detected at -10 mV and at 20 mV reached its maximum with a t1/2 = 12 ms. For IK, GK = 200 microseconds/cm2. These channels can be activated during action potentials and play a role in the excitation and contraction of vascular smooth muscle cells.

Calcium-sensitive cellular and subcellular transport of sodium, potassium, magnesium, and calcium in sodium-loaded vascular smooth muscle. Electron probe analysis

Electron probe x-ray microanalysis of the composition of rabbit portal anterior mesenteric vein smooth muscle was performed following sodium loading and washout into sodium-free lithium solutions. Sodium and lithium were also measured with atomic absorption spectrophotometry. Cellular uptake of sodium and loss of potassium during sodium loading were much faster at high (37 degrees C) than at low (2 degrees C) temperature, as was the passive ouabain-resistant uptake of potassium during lithium washout. The loss of sodium at 2 degrees C into lithium solution consisted of two components: a rapid efflux that was complete by 30 minutes, and a slow component that required at least 24 hours for completion. The amount of sodium lost through the first component (approximately 200-300 mmol/kg dry weight) was relatively independent of the amount of sodium loading. The loss of cellular sodium at 2 degrees C, after 30 minutes, was accompanied by a gain of cellular lithium. Ouabain-resistant sodium loss and lithium and potassium uptake were markedly accelerated at 37 degrees C; sodium loss was complete (1200 mmol sodium/kg dry weight lost) by 30 minutes of washout. Sodium-loaded cells also lost chloride ion and gained magnesium during sodium efflux at 37 degrees C. Mitochondrial and nuclear sodium and potassium were correlated with the respective cytoplasmic concentrations during both sodium loading and sodium washout, indicating the relatively rapid equilibration of the monovalent ions between the cytoplasm and organelles. Calcium-free solutions markedly inhibited the ouabain-resistant sodium and chloride ion effluxes and potassium influx in muscles incubated, after sodium loading, in lithium solutions at 37 degrees C. These fluxes could be restored to near normal values by 0.2 mM calcium. The calcium sensitivity of the ouabain-resistant sodium, potassium, and chloride ion fluxes observed in this and other studies raises the possibility that some abnormalities of monovalent ion transport observed in cells of hypertensives are secondary to changes in cellular calcium.

New views of smooth muscle structure using freezing, deep-etching and rotary shadowing

Freezing, deep-etching and rotary shadowing techniques have been applied to study smooth muscle ultrastructure. The results show some new aspects of intracellular and extracellular connections, interior views of the sarcoplasmic reticulum showing a luminal content, coated pits and vesicles, contractile filaments and other organelles in smooth muscle.