Effects of external K+ on depletion-induced Ca2+ entry in rat ileal smooth muscle

Effects of inhibitors for tyrosine kinase and non-selective cation channel on capacitative Ca(2+) entry in rat ileal smooth muscle

The effects of tyrosine kinase inhibitors and non-selective cation channel blockers on capacitative Ca(2+) entry were examined in the presence of methoxyverapamil in rat ileal smooth muscles. In Ca(2+)-free solution, carbachol or caffeine produced a rapid contraction mediated by Ca(2+) release from the stores (Ca(2+)-release response), and then led to Ca(2+) depletion of the stores. Subsequently, reintroduction of Ca(2+) caused a transient contraction due to capacitative Ca(2+) entry. Tyrosine kinase inhibitors, genistein and tyrphostin 47 but not herbimycin A, suppressed the responses to Ca(2+)-reintroduction much greater than Ca(2+)-release responses to carbachol or caffeine. Similar inhibitory effects on the responses to Ca(2+)-reintroduction were obtained with daidzein and tyrphostin A1, respective inactive analogue of genistein and tyrphostins. After continuous depletion of the stores with thapsigargin, Ca(2+)-reintroduction produced a sustained contraction, which was inhibited by these agents to different extents, but not by herbimycin A. In beta-escin-treated skinned muscles, genistein slightly reduced Ca(2+)-induced contraction. In fura-2-loaded tissues, SK&F 96365 inhibited contractile and [Ca(2+)](i) responses to Ca(2+)-reintroduction but minimally affected Ca(2+)-release responses. Tetrandrine suppressed both responses to Ca(2+)-reintroduction and to Ca(2+)-release. These results suggest that genistein and tyrphostin 47 inhibit capacitative Ca(2+) entry through an inhibition of Ca(2+) entry channels rather than tyrosine kinase. SK&F 96365, but not tetrandrine, seems to selectively inhibit the contractile responses to capacitative Ca(2+) entry in rat ileal smooth muscles.

Numerical simulation of motility patterns of the small bowel. II. Comparative pharmacological validation of a mathematical model

A model of a locus of the small bowel, described earlier by the authors (Miftakhov et al., 1999) was validated in a comparison of the results of numerical simulations of pharmacological compounds to their effects in biological studies. The actions of the following four classes of drugs were simulated, those: (i) acting on the sarcoplasmic reticulum, (ii) altering the permeability of L- and T-type Ca(2+)channels on the smooth muscle membrane, (iii) motilides, and (iv) benzodiazepines. The strong qualitative resemblance between the theoretical and experimental results supports the robustness of the model.

Depolarization-mediated inhibition of Ca(2+) entry in endothelial cells

The effect of extracellular Cl(-) in regulating ACh-induced Ca(2+) entry into freshly isolated rabbit aortic endothelial cells was studied using Ca(2+)-sensitive fluorescence microscopy and patch-clamp electrophysiology. After ACh caused transient Ca(2+) release in Ca(2+)-free medium, readdition of 3 mM Ca(2+) to the bath maintained Ca(2+) entry. Removal of extracellular Cl(-) abolished the plateau phase in Ca(2+) signal and inhibited divalent cation entry. However, in the presence of the K(+) ionophore valinomycin, removal of Cl(-) had no effect on the Ca(2+) plateau. Under current-clamp conditions, substitution of gluconate for Cl(-) induced membrane depolarization. Under voltage clamp, with CsCl in the pipette, ACh activated a slowly developing Cl(-) current, which was blocked by SITS and 5-nitro-2-(3-phenylpropylamino)benzoic acid. Varying the membrane potential by utilizing different extracellular K(+) concentrations in the presence of 5 microM valinomycin demonstrated that depolarization blocked ACh-stimulated Mn(2+) entry. These data suggest that ACh-induced Ca(2+) entry in freshly isolated endothelial cells requires the presence of extracellular Cl(-) to maintain a polarized membrane potential.

Mechanisms involved in the cellular calcium homeostasis in vascular smooth muscle: calcium pumps

The regulation of cytosolic Ca2+ homeostasis is essential for cells, and particularly for vascular smooth muscle cells. In this regulation, there is a participation of different factors and mechanisms situated at different levels in the cell, among them Ca2+ pumps play an important role. Thus, Ca2+ pump, to extrude Ca2+; Na+/Ca2+ exchanger; and different Ca2+ channels for Ca2+ entry are placed in the plasma membrane. In addition, the inner and outer surfaces of the plasmalemma possess the ability to bind Ca2+ that can be released by different agonists. The sarcoplasmic reticulum has an active role in this Ca2+ regulation; its membrane has a Ca2+ pump that facilitates luminal Ca2+ accumulation, thus reducing the cytosolic free Ca2+ concentration. This pump can be inhibited by different agents. Physiologically, its activity is regulated by the protein phospholamban; thus, when it is in its unphosphorylated state such a Ca2+ pump is inhibited. The sarcoplasmic reticulum membrane also possesses receptors for 1,4,5-inositol trisphosphate and ryanodine, which upon activation facilitates Ca2+ release from this store. The sarcoplasmic reticulum and the plasmalemma form the superficial buffer barrier that is considered as an effective barrier for Ca2+ influx. The cytosol possesses different proteins and several inorganic compounds with a Ca2+ buffering capacity. The hypothesis of capacitative Ca2+ entry into smooth muscle across the plasma membrane after intracellular store depletion and its mechanisms of inhibition and activation is also commented.

Different actions in the rat prostatic and epididymal vas deferens of cyclopiazonic acid or ryanodine on noradrenaline-induced contractions

The effects of ryanodine, cyclopiazonic acid (CPA), and nifedipine on noradrenaline (NA)-induced contractions were investigated to characterize the role of the sarcoplasmic reticulum (SR) in the epididymal and prostatic parts of the rat vas deferens. In the epididymal part, NA (0.1, 1, and 100 microM) evoked marked rhythmic contractions superimposed on a tonic response. NA (100 microM) evoked biphasic tonic contractions consisting of a fast (initial) component and delayed secondary components. Nifedipine (1 microM) suppressed the rhythmic activity and the contractions to low NA concentrations and markedly reduced the components of the response to NA (100 microM). Contractions of the epididymal part to NA (0.1, 1, and 100 microM) were not blocked by ryanodine (1-30 microM) or CPA (1-30 microM). The secondary component in the response to NA (100 microM) was enhanced by CPA (> or =10 microM). Thus in the epididymal part, NA stimulates contraction predominantly by mobilizing extracellular calcium. However, a residual nifedipine-insensitive contraction to NA (100 microM) was observed and was not blocked by ryanodine (30 microM) or CPA (30 microM). In the prostatic part, NA evoked mainly tonic contractions. The response to NA (100 microM) consisted of three distinct components. Nifedipine (1 microM) reduced the contractions to low concentrations of NA (0.1 and 1 microM) and all three components of the response to NA (100 microM). Contractions of the prostatic part to low concentrations of NA (0.1 and 1 microM) were not blocked by CPA (30 microM) or ryanodine (30 microM). The components of the response to NA (100 microM) were affected differently by the drugs. Ryanodine (17-30 microM) or CPA (1-30 microM) suppressed the initial component and reduced the second component. The third component was largely unaffected by CPA but reduced by ryanodine. In the additional presence of nifedipine (1 microM), the residual components of NA (100 microM) response were markedly reduced and the contractions to low concentrations of the agonist virtually abolished. These results suggest that NA contracts the prostatic part by mobilizing both extra- and intracellular calcium. These results show that NA-induced contractions of the epididymal and prostatic parts of the rat vas deferens differ in sensitivity to ryanodine or CPA. The results suggest that, during stimulation of the epididymal part, the SR functions mainly to buffer calcium entering through nifedipine-sensitive voltage-gated calcium channels. In contrast, in the prostatic part, the SR serves mainly as a source of calcium and contributes more to contractions evoked by higher concentrations of the agonist.

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.

Electromechanical coupling in human vas deferens: effects of agents that modulate intracellular release of calcium

1. The effects of ryanodine, cyclopiazonic acid (CPA) and caffeine on electromechanical coupling in human vas deferens were investigated. 2. High [K+]o (120 mM) evoked nifedipine-sensitive contractions of longitudinal and circular muscle which consisted of initial and secondary components. 3. Exposures to ryanodine (< or =10 microM) or CPA (< or = 3 microM) induced a change of basal tension, and higher doses (30 microM) induced intermittent rhythmic contractions of both muscle types in the quiescent tissue. In the presence of the drugs, contraction to high [K+]o was preceded by marked rhythmic activity. 4. In circular muscle, ryanodine (1-30 microM) or CPA (1-30 microM) reduced both components of contractions to high [K+]o. In longitudinal muscle, the drugs enhanced the initial component and prolonged the secondary component. High doses (> or = 10 microM) produced variable effects on the initial component. 5. Caffeine (20 mM) reliably contracted longitudinal, but not circular muscle. Pre-exposures to caffeine enhanced both components in the post-caffeine contractions of circular muscle to high [K+]o. In longitudinal muscle, only the initial component (post-caffeine) was enhanced. 6. Contractions evoked in longitudinal muscle by caffeine were not blocked by ryanodine (30 microM) or CPA (30 microM). However, the enhancement of post-caffeine contractions to high [K+]o was inhibited. 7. These results show that ryanodine and CPA produced comparable effects on the excitability of longitudinal and circular muscle in the quiescent tissue, but electromechanical coupling was affected differently. The findings suggest that the muscle types utilize different mechanisms to regulate elevations in cytosolic Ca2+ during stimulation. 8. Electromechanical coupling in both muscle types involves Ca2+ influx via nifedipine-sensitive voltage-operated calcium channels and activation of ryanodine-sensitive calcium-induced calcium release from the sarcoplasmic reticulum (SR). In longitudinal muscle, the SR also buffers increases in cytosolic Ca2+ via a pharmacologically distinct Ca2+ compartment (caffeine releasable but ryanodine/CPA-insensitive). In circular muscle, the SR (ryanodine/CPA-sensitive) serves mainly in the regulation of excitability of the quiescent tissue.