Ca(2+)-induced inhibition of 45Ca2+ influx and Ca2+ current in smooth muscle of the rat vas deferens

A new view of K+ -induced contraction in rat aorta: the role of Ca2+ binding

Strong, K+ -induced contractions of rat aorta in Ca-free, Mg-free media were not accompanied by increased intracellular calcium concentration, [Ca2+](i), whereas such contractions in the presence of the divalent cations were correlated with rising [Ca2+](i) as assessed by fura-2. At the same time, calcium channel blockers, a modulator of Ca2+-binding proteins, and a modulator of actin polymerization, inhibited all types of K+ -induced contractions. Increasing the K+ in isotonic medium evoked a rise of (45)Ca2+ binding to the plasma membrane of freshly isolated aortic cells. Although Ca2+ -dependent events underlie the mechanism of K+ -induced vascular contractions in both the presence and absence of Ca2+, in contrast to the view that [Ca2+](i) is a key regulator of excitation-contraction coupling in smooth muscle, we suggest that the modulation of Mg2+ -dependent Ca2+ binding, probably within/at the L-type calcium channel by K+, is a trigger for aortic contraction. This Ca2+ binding may then activate actin-myosin interaction.

Regulation of cardiac and smooth muscle Ca(2+) channels (Ca(V)1.2a,b) by protein kinases

High voltage-activated Ca(2+) channels of the Ca(V)1.2 class (L-type) are crucial for excitation-contraction coupling in both cardiac and smooth muscle. These channels are regulated by a variety of second messenger pathways that ultimately serve to modulate the level of contractile force in the tissue. The specific focus of this review is on the most recent advances in our understanding of how cardiac Ca(V)1.2a and smooth muscle Ca(V)1.2b channels are regulated by different kinases, including cGMP-dependent protein kinase, cAMP-dependent protein kinase, and protein kinase C. This review also discusses recent evidence regarding the regulation of these channels by protein tyrosine kinase, calmodulin-dependent kinase, purified G protein subunits, and identification of possible amino acid residues of the channel responsible for kinase regulation.

Calcium sparks in smooth muscle

Local intracellular Ca(2+) transients, termed Ca(2+) sparks, are caused by the coordinated opening of a cluster of ryanodine-sensitive Ca(2+) release channels in the sarcoplasmic reticulum of smooth muscle cells. Ca(2+) sparks are activated by Ca(2+) entry through dihydropyridine-sensitive voltage-dependent Ca(2+) channels, although the precise mechanisms of communication of Ca(2+) entry to Ca(2+) spark activation are not clear in smooth muscle. Ca(2+) sparks act as a positive-feedback element to increase smooth muscle contractility, directly by contributing to the global cytoplasmic Ca(2+) concentration ([Ca(2+)]) and indirectly by increasing Ca(2+) entry through membrane potential depolarization, caused by activation of Ca(2+) spark-activated Cl(-) channels. Ca(2+) sparks also have a profound negative-feedback effect on contractility by decreasing Ca(2+) entry through membrane potential hyperpolarization, caused by activation of large-conductance, Ca(2+)-sensitive K(+) channels. In this review, the roles of Ca(2+) sparks in positive- and negative-feedback regulation of smooth muscle function are explored. We also propose that frequency and amplitude modulation of Ca(2+) sparks by contractile and relaxant agents is an important mechanism to regulate smooth muscle function.

Pharmacomechanical coupling in rat vas deferens: effects of agents that modulate intracellular release of calcium and protein kinase C activation

The effects of agents that modulate intracellular release of calcium and protein kinase C (PKC) activation on noradrenaline (NA)-induced contractions of epididymal vas deferens in calcium-free/EGTA (1 mM) medium were investigated. NA (100 microM) or methoxamine (100 microM) evoked repeatable contractions. Clonidine (100-300 microM) was ineffective. The contractions to NA were reduced by procaine (1-10 mM) but not by thapsigargin (0.1-30 microM), ryanodine (1-30 microM) or TMB-8 (1-30 microM). Contractions to cumulative additions of NA (1-100 microM) were enhanced in the presence of cyclopiazonic acid (10 & 30 microM) but not ryanodine (10 & 30 microM). Sequential contractions to NA were not blocked by PKC inhibitors, calphostin C (1 microM) or Ro 31-8220 (1-30 microM) but were reduced by H-7 (1-30 microM), a broad spectrum protein kinase inhibitor. Although RT-PCR experiments detected mRNA for some Ca2+-dependent/DAG-activated and Ca2+-independent/DAG-activated PKC isoforms in epididymal vas deferens, the PKC activators, phorbol 12, 13-dibutyrate (100 microM) or phorbol 12-myristate 13-acetate (100 microM) failed to activate the tissues in calcium-free medium but enhanced subsequent contractions to NA. These results indicate a limited role for intracellular calcium stores and phorbol ester/DAG-sensitive PKC isoforms in NA-induced contraction of epididymal rat vas deferens in calcium-free medium. The results suggest that pharmacomechanical coupling triggered by NA may involve the sensitization of contractile myofilaments to Ca2+ or a Ca2+-independent mechanism. The possible involvement of Ca2+-independent/DAG-insensitive PKC isoforms and agonist-dependent but PKC-independent sensitization pathway is discussed.

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.

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.