Effects of inhibitors of calcium-induced calcium release on receptor-mediated responses of smooth muscles and platelets.

Abstract

Ca-induced Ca release (CICR) first found in the sarcoplasmic reticulum of skeletal muscle1 '2 is an interesting biological phenomenon, but it does not appear to play a major role in normal excitation-contraction coupling in the amphibian skeletal muscle.3>>4> However, it might play an important physiological role in other kinds of cells such as medaka eggs, as suggested by propagation of `calcium wave' upon their fertilization.5> If a response of some cells is mediated by the CICR mechanism, it should be inhibited by inhibitors of CICR. Therefore, to examine whether or not inhibitors of CICR inhibit the response constitutes a good screening test for the possibility of involvement of the CICR mechanism in evoking the response in question. Indeed, the absence of inhibition of normal excitation-contraction coupling of amphibian skeletal muscle by inhibitors of CICR°>,7) furnished strong evidence against physiological role of the mechanism in this tissue. In the present article, the effect of procaine and adenine, two different inhibitors of CICR,6) 8 ° was examined on contractions of smooth muscles evoked by transmitters and on shape changes of platelets. Thin bundles of smooth muscle fibers were dissected from taenia caeci or portal vein from the guinea pig. They were suspended in a trough in which solutions could be exchanged rapidly. Isometric tension was monitored by a strain-gauge transducer (UL-2, Shinkoh, Tokyo) and displayed on a pen recorder. Contractions were evoked by applying caffeine or neurotransmitters, acetylcholine or carbachol in taenia caeci and noradrenaline in portal vein. To see the result of Ca release from an intracellular store without participation of changes in membrane potential and of Ca entry from the extracellular fluid, experiments were done in a medium in which all Na ions were replaced by K ions and 4 mM EGTA (ethyleneglycolbis-(,Q-aminoethylether)-N-N'-tetraacetic acid) was present. Platelets were obtained from arterial blood drawn through a plastic cannula inserted into the femoral artery of albino rabbits under ether anesthesia. Blood was immediately mixed with 1/9 volume of sodium citrate solution (final concentration about 11 mM) and platelet-rich plasma obtained as the supernatant by centrifuging the citrated blood at 230 X g for 7 min. One volume of the platelet-rich plasma was mixed with 4 volume of a buffer (Na2SO4 or K2SO4 t)

Calcium-induced calcium release mechanism in guinea pig taenia caeci.

“…It is difficult to estimate the peak Ca 2+ level thus attained without knowing the concentration and the kinetics of the intracellular Ca binding sites as well as the Ca z+ dependence of Ca extrusion mechanisms in both the Ca store and the plasma membrane. However, the results obtained in skinned fibers seem to be in accordance with the finding that the minimum concentration of caffeine for a contracture of intact taenia caeci is 1-5 mM at -20"C (Ito and Kuriyama, 1971;Yagi et al, 1985).…”

Pharmacological Modulation of Sarcoplasmic Reticulum Function in Smooth Muscle

“…Similarly, in the absence of extracellular Ca 2ϩ , procaine (1 mM) inhibits Cch-induced (1-1000 M) guinea pig taenia caeci contractile activity (Yagi et al, 1985). The same concentration of procaine also inhibits Ach-induced (10 M) contractile activity of the porcine coronary artery in absence of extracellular Ca 2ϩ .…”

“…For instance, in the porcine coronary artery, procaine (5 mM) inhibits caffeine (20 mM)-induced contractile activity of isolated strips in the absence of extracellular Ca 2ϩ (Itoh et al, 1982a) and inhibits (1-10 mM) the associated reduction in cellular Ca 2ϩ content in saponin-permeabilized smooth muscle cells (Ueno et al, 1987). Likewise, in guinea pig taenia caeci, procaine (1 mM) inhibits caffeine-induced (1-25 mM) contractile activity in the absence of extracellular Ca 2ϩ (Yagi et al, 1985), consistent with the inhibition of CICR rate by procaine (millimolar concentrations) in fura-2-loaded saponin-permeabilized smooth muscle fiber bundles (Iino, 1989).…”

Guanine nucleotide‐ and inositol 1,4,5‐trisphosphate‐induced calcium release in rabbit main pulmonary artery.

“…Procaine (20 mM) completely abolished the contraction induced by 20 mM-caffeine. However, 40 /LM-InsP3 (Yagi et al 1985;van Breemen et al 1986) and 100 /SM-GTPyS still induced substantial contraction in the presence of 20 mM-procaine. Although the InsP3-induced contraction was slightly smaller in the presence than in the absence of procaine, the proportion between contractions induced by InsP3 and GTPyS was not significantly changed (data not shown).…”