Calcium distribution and exchange in the rat uterus

The calcium content and distribution of the rat uterus were determined employing flame photometry and Ca⁴⁵ determinations. The total uterine calcium concentration was found to be 2.25 millimoles (mmoles) per kilogram wet weight, 0.45 of which was inexchangeable. The exchangeable Ca could be divided into 0.8 mmole/kg wet weight extracellular and 1.0 mmole/kg wet weight intracellular. The concentration of ionic Ca in rat serum was obtained by equilibrium dialysis as 1.5 mM or 53 % of the total serum Ca. The observed Ca distribution required that its active transport be postulated, since the membrane was shown to be permeable to Ca and the internal Ca concentration was far below its electrochemical equilibrium value. Metabolic inhibition by iodoacetate or dinitrophenol caused a net Ca uptake, but cooling to 4°C and ouabain did not. Iodoacetate did not affect the Ca⁴⁵ efflux, but did increase the influx, suggesting that active Ca transport is accomplished by an exclusion mechanism. In experiments with varied external sodium concentrations, no evidence was obtained that sodium competes with calcium for inward transport. Cellular Ca binding was measured under conditions of prolonged metabolic inhibition, which abolished both active transport and the membrane potential. The association constants obtained were compatible with intracellular Ca binding to proteins, but insufficient to account for the low level of intracellular ionic Ca believed essential for relaxation. Hence metabolically dependent intracellular Ca binding was postulated. The Ca⁴⁵ efflux was slowed down by Ca-free efflux media. The presence of Sr or EDTA could completely prevent this decrease in efflux rate, and Ba could partly prevent it. Changes in Mg and Na concentration did not affect the rate of Ca⁴⁵ efflux. A model to explain Ca exchange across smooth muscle membranes has been proposed.     

The effects of thyrotropin-releasing hormone and potassium depolarization on phosphoinositide metabolism and cytoplasmic calcium in bovine pituitary cells

Addition of thyrotropin-releasing hormone (TRH) (10 nM to 10 microM) to bovine anterior pituitary cells labelled with [3H]inositol decreased the radioactivity in inositol-containing lipids and increased it in inositol phosphates. TRH also increased the cytoplasmic calcium concentration biphasically. At TRH concentrations below 10 nM, the increase was sustained and sensitive to inhibitors of calcium influx through voltage-gated channels, whereas concentrations over 10 nM elicited in addition a rapid transient increase in calcium, which was relatively insensitive to such inhibition. Incubation of the cells in medium containing 25 mM KCl increased the cytoplasmic calcium concentration by stimulating influx through voltage-gated channels, and markedly enhanced the initial transient increase of calcium seen at TRH concentrations above 10 nM. It did not affect the generation of InsP3 and it also enhanced the calcium response to ionomycin. It is suggested that stimulation of calcium entry through voltage-gated channels can increase the amount of calcium available for mobilisation by TRH.     

Role of depolarization and calcium in contractions of canine trachealis from endogenous or exogenous acetylcholine

The relationships of the electrical to the mechanical responses of the canine trachealis muscle during stimulation of its cholinergic nerves or exposure to exogenous acetylcholine were recorded in the single or the double sucrose gap. At 27 degrees C, the responses to a train of stimuli consisted of a transient depolarization excitatory junction potential of 10-30 mV followed by fading oscillations and contractions. When stimulus parameters were varied in the single sucrose gap, contractions were more closely associated with the occurrence of and varied in duration with the oscillations rather than with the amplitude of the EJP. Acetylcholine superfused at a concentration of 10(-6) M for 30 s caused a prolonged depolarization of 10-20 mV, but a much larger contraction than could be elicited by nerve stimulation. None of the responses to acetylcholine was significantly affected by the Ca channel antagonists, nifedipine, nitrendipine, or verapamil in Ca channel blocking concentrations. When tissues were exposed to a Ca-free medium, the excitatory junction potentials and oscillations rapidly disappeared, but the electrical and mechanical responses to acetylcholine persisted and only gradually disappeared with repetitive exposures. Furthermore, in a medium with normal Ca2+ in the double sucrose gap, depolarization by 10-15 mV with an applied current caused no contraction, and repolarization to the normal membrane potential during acetylcholine-induced contraction caused no relaxation. Tetraethylammonium ion (20 mM) depolarized the membrane, increased membrane resistance, and enhanced the secondary oscillations and contractions after field stimulation. No other K(+)-channel blocker tested (Ba2+, apamin, 4-aminopyridine, glibenclamide, charybdotoxin) had the effect of prolonging secondary oscillations.(ABSTRACT TRUNCATED AT 250 WORDS)     

Effect of lanthanum on the turnover of calcium in rat uterus.

Previous investigations suggest that lanthanum might enter uterine smooth muscle cells and work as intracellular calcium displacing agent. The present investigation had been carried out in order to confirm if lanthanum develops an intracellular effect. Experiments show that lanthanum brings about a marked increase of the intracellular calcium; the comparison of the uptake and of the wash-out curve of 45Ca shows that lanthanum induces a lowering of the rapid phase of 45Ca release from rat uterus, while the uptake of the labelled ion is not modified or is even enhanced. The present data demonstrate that the action of lanthanum in rat uterus is limited to the cell membrane, whose calcium extruding properties are inhibited.     

The influence of phosphorylable molecules on calcium transfer in the rat ileum

Amino or hydroxylated molecules have been tested from both points of view of their effect on calcium absorption in ileal loops, and of their ability to be phosphorylated. Some molecules, very effective in enhancing calcium absorption, are also highly phosphorylable: aspartic and glutamic acids and creatine. Other molecules, virtually ineffective, are not phosphorylable: L-alanine, L-valine, asparagine and glutamine. Ileal mucosa extract induces transphosphorylation from ATP to lysine and arginine, the amino acids the most potent in increasing calcium absorption. The observation that both isomers of the same ose (D- and L-xylose) or that a non absorbable polyoside (raffinose) increase calcium absorption may be linked to the fact that all are phosphorylated $\text{in vitro}$ by a purified ileal phosphatase. Injection of either sorbitol or creatine into an ileal loop induces formation of their phosphorylated derivatives, thereby providing circumstantial evidence for a direct relationship between the phosphorylation ability of these molecules and their involvement in calcium transfer mechanisms.     

The influence of extracellular calcium on microvascular tone in the rat cremaster muscle

In vivo responses of arterioles and venules to changes in bath calcium concentrations were observed in the cremaster muscle of male Sprague-Dawley rats. Small arterioles (2A, 3A) initially exposed to a solution containing calcium (2.55 mM) significantly dilated in response to a 0-calcium bath. Reexposure to calcium (greater than 0.65 mM) caused 2A and 3A arterioles to constrict to diameters similar to the initial control values. In contrast, large arterioles (1A) and all venules (1V, 2V, 3V) were unresponsive to exposure to a 0-calcium solution or to reexposure to calcium (0.65-5.10 mM). Treatment with mefenamic acid (10 micrograms/ml), a prostaglandin synthesis inhibitor, produced marked constriction of arterioles but not of venules, suggesting the involvement of endogenous vasodilator prostaglandins in the regulation of resting diameters of arterioles. In the presence of mefenamic acid, 1A arterioles dilated when exposed to a 0-calcium solution and constricted back to control diameters following reintroduction of calcium into the bath. These data demonstrate heterogeneity in the responsiveness of cremasteric microvessels to changes in extracellular calcium. The small arterioles were most responsive to calcium. The lack of response by the largest arterioles appears to be due to the dilator influences of endogenous prostaglandins.     

The influence of calcium on ANF release in the isolated rat atrium.

We developed an in vitro model of the isolated, perfused rat atrium with which to examine the mechanisms linking muscular stretch to atrial natriuretic factor (ANF) secretion. It was shown that an increase in atrial pressure causing distension of the atria is associated with a rise in ANF secretion correlating with the degree of pressure load. Pressure-induced ANF secretion is enhanced by the calcium blocker nifedipine or omission of calcium from the perfusion buffer. The changes in atrial volume in response to a given pressure load are also more pronounced in the absence of calcium or following the addition of the calcium blocker. These data suggest that in nonbeating atria, stretch-induced ANF secretion does not rely on calcium influx.     

Rapid calcium exchange for protons and potassium in cell walls of Chara

Net fluxes of Ca²⁺, H⁺ and K⁺ were measured from intact Chara australis cells and from isolated cell walls, using ion-selective microelectrodes. In both systems, a stimulation in Ca²⁺ efflux (up to 100 nmol m^?2 s^?1, from an influx of ～40 nmol m^?2 s^?1) was detected as the H⁺ or K⁺ concentration was progressively increased in the bathing solution (pH 7.0 to 4.6 or K⁺ 0.2 to 10mol m^?3, respectively). A Ca²⁺ influx of similar size occurred following the reverse changes. These fluxes decayed exponentially with a time constant of about 10 min. The threshold pH for Ca²⁺ efflux (pH 5.2) is similar to a reported pH threshold for acid-induced wall extensibility in a closely related characean species. Application of NH₄⁺ to intact cells caused prolonged H⁺ efflux and also transient Ca²⁺ efflux. We attribute all these net Ca²⁺ fluxes to exchange in the wall with H⁺ or K⁺. A theoretical treatment of the cell wall ion exchanges, using the ‘weak acid Donnan Manning’ (WADM) model, is given and it agrees well with the data. The role of Ca²⁺ in the cell wall and the effect of Ca²⁺ exchanges on the measured fluxes of other ions, including bathing medium acidification by H⁺ efflux, are discussed.