Calcium channel antagonist binding and pharmacology in rat uterine smooth muscle

Store operated Ca2+ entry dependent contraction of coronary artery smooth muscle: inhibition by peroxide pretreatment

The sarco/endoplasmic reticulum (SER) Ca(2+) pool is refilled by the SER Ca(2+) pump (SERCA) using cytosolic Ca(2+) and/or extracellular Ca(2+) entering the cell. The effects of the SERCA pump inhibitor cyclopiazonic acid (CPA) were studied in pig coronary artery smooth muscle using two protocols. In protocol A, the SERCA pump was inhibited by adding CPA to cells/tissues in Ca(2+)-containing solution, whereas in protocol B, CPA was added to cells/tissues in Ca(2+)-free solution, followed by reintroduction of extracellular Ca(2+). Addition of CPA increased cytosolic Ca(2+) in cultured smooth muscle cells and elicited contraction in de-endothelialized coronary arteries in both protocols. Based on pharmacological experiments, the CPA-induced contraction of de-endothelialized arteries in protocol B resulted from store operated Ca(2+) entry (SOCE). Reactive oxygen species such as peroxides are known to damage the SERCA pump in this tissue. Consistently, CPA-induced contractions were decreased in arteries pre-treated with hydrogen peroxide in protocol A. However, this pretreatment also decreased the force of contraction due to SOCE in protocol B, suggesting that it closed SOCE. We propose that the closure of SOCE triggered by exposure to reactive oxygen species may be a protective mechanism, so that Ca(2+) entry by this pathway is disallowed when SERCA is damaged in pathologies such as ischemia-reperfusion.

Sodium-calcium exchange mediated contraction in left anterior descending and left ventricular branch arteries

We tested the hypothesis that the de-endothelialized artery rings from the left anterior descending (LAD) coronary artery and its left ventricular branch (LVB) differ in their contractile responses to Na⁺–Ca²⁺-exchanger (NCX) mediated Ca²⁺-entry, muscarinic receptor activation with carbachol, and sarco/endoplasmic reticulum Ca²⁺ pump (SERCA) inhibition with thapsigargin. In LVB, the force of contraction (in N/g tissue) produced by the NCX mediated Ca²⁺-entry (17.5 ± 1.4) and carbachol (18 ± 1.5) was only slightly smaller than that due to membrane depolarization with KCl (24.0 ± 1.0). In contrast, in LAD the force of contraction produced with NCX (8.7 ± 0.7) and carbachol (6.1 ± 1.1) was much smaller than with KCl (15.7 ± 0.7). Thapsigargin also contracted LVB with greater force than LAD. When isolated microsomes were used, the binding to the muscarinic receptor antagonist quinuclidinyl benzilate was greater in LVB than in LAD. Microsomes were also used for Western blots. The intensities of signals for both SERCA and NCX were greater in LVB than in LAD. These biochemical observations were consistent with the contractile experiments. Thus, it appears that the differences between LAD and the resistance arteries may begin as early as LVB.

Expression of the internal calcium pump in pregnant rat uterus

Uterine contraction to agents such as oxytocin during labour may utilize Ca2+ sequestered by Ca2+ pump into the sarcoplasmic reticulum (SR). Uterus expresses the SR Ca2+ pump gene SERCA2 as the mRNA splice which encodes the protein SERCA2b. The expression of SERCA2 mRNA and protein was monitored in uteri of day 15 pregnant and delivering rats. The ratio of SERCA2 mRNA to 28S RNA increased by 20% from day 15 to delivery. SERCA2 protein examined by two antibodies increased by 54-55%. Ca2+ dependent acylphosphate intermediate of 115 kD corresponding to SERCA2 also increased 72% during this period. Thus, there is an increase in the SERCA2 expression from day 15 to delivery.

Evidence for the presence of regional differences in the calcium antagonist receptors in lower urinary tract smooth muscle

(+)-[3H]PN 200-100 (a dihydropyridine calcium channel antagonist) was utilized to characterize calcium channel binding sites in rabbit bladder dome, bladder base, and urethra. Specific binding of (+)-[3H]PN 200-110 to membrane particulates was saturable, reversible, linear to protein concentration, and of high affinity. The density of (+)-[3H]PN 200-110 binding sites (Bmax values in fmol/mg of protein) and the affinity constants for (+)-[3H]PN 200-110 (KD value in pM) in urethra, bladder dome and bladder base were 64.1 +/- 7.8 and 179 +/- 31; 21.9 +/- 3.0 and 213 +/- 36; and 18.8 +/- 4.2 and 140 +/- 28, respectively. Agonists and antagonists inhibited (+)-[3H]PN 200-110 binding with Ki values in the following rank order: nitrendipine less than nifedipine less than niguldipine much less than Bay K 8644 much less than verapamil. Although carbachol-induced contractile responses were 20-30 times smaller in muscle strips from urethra than from bladder base or bladder dome, KCl-induced contractions were only 3-4 times smaller in urethra than in bladder tissues. Nifedipine inhibited carbachol-induced contractions in urethra, bladder dome, and bladder base by 76%, 64%, and 60%, respectively, and completely inhibited KCl-induced contractions in all three tissues. IC50 values for nifedipine inhibition of both carbachol- and KCl-induced contractions were significantly smaller in urethra than in bladder base or bladder dome. Nitrendipine, niguldipine and verapamil inhibited urethral contractions induced by carbachol and KCl to the same degree as did nifedipine. The IC50 values, obtained from functional studies, for calcium channel antagonists were in good agreement with Ki values obtained from binding studies.(ABSTRACT TRUNCATED AT 250 WORDS)

Evidence for the mechanism of the inhibitory action of jatrophone in the isolated rat uterine muscle

1. Jatrophone (JAT), a diterpene isolated from the plant Jatropha elliptica (1-300 microM), caused a concentration-dependent relaxation effect against acetylcholine (Ach)-oxytocin (Ot)- and KCl-induced uterine sustained contraction. The relative potency order was: Ach greater than Ot greater than KCl. 2. The relaxant effect of JAT was not modified by phorbol ester, forskolin, MIX, TMB-8 and W-7. The increase concentration of calcium (0.2-2 mM) in the medium did not reverse the inhibitory effect caused by JAT. 3. Pre-incubation of the preparations with JAT (16-32 microM) for 20 min, caused a concentration-dependent inhibition of KCl-induced contractile response. At 30 microM, JAT inhibited in an apparently non-competitive manner CaCl2-induced contraction in K+-depolarized preparations. High concentrations of JAT (100 microM) also caused a time-dependent relaxation in CaCl2-induced sustained uterine contraction (T1/2 = approx. 15 min). 4. JAT (30 microM) inhibited the dihydropyridine calcium channel agonist Bay K 8644-induced uterine contraction in an apparently non-competitive fashion, while verapamil (0.1 microM) caused an rightward displacement of Bay K 8644 contraction and marked inhibition of the maximal response.

Endothelin induces two types of contractions of rat uterus: phasic contractions by way of voltage-dependent calcium channels and developing contractions through a second type of calcium channels

Effects of endothelin on nonvascular smooth muscle have been examined using rat uterine horns and two modes of endothelin action have been revealed. Endothelin (0.3 nM) caused rhythmic contractions of isolated uterus in the presence of extracellular calcium. The rhythmic contractions were completely inhibited by calcium channel antagonists. These characteristics of endothelin-induced contractions were very similar to those induced by oxytocin. Binding assays using 125I-endothelin showed that endothelin and the calcium channel blockers did not compete for the binding sites. However, endothelin was unique in that it caused, in addition to rhythmic contractions, a slowly developing monophasic contraction that was insensitive to calcium channel blockers. This developing contraction became dominant at higher concentrations of endothelin and was also calcium dependent.

Interactions between forskolin, calcium and Bay K 8644 in K+-depolarized rat uterine muscle

1. Forskolin (FORS) (0.03 to 10 microM) caused a concentration-dependent rightward shifts of the dose-response curves to CaCl2 in the isolated K+-depolarized rat uterus, with apparent, pA2 value of 8.3, but the slope was different from unity (0.55). 2. In presence of Bay K 8644 (10 and 100 nM), the pA2 values for FORS against CaCl2 contractions were reduced in a concentration-dependent manner and increased the slopes of Schild plots to values no longer different from unity. 3. In presence of low Ca2+ concentration (0.22 nM), the potency of FORS in antagonizing CaCl2 contractions was reduced in about 2-fold. 4. FORS (0.3-3 microM) caused a dose- and time-dependent relaxation of sustained contraction induced by CaCl2 (0.5 mM). 5. FORS (0.3 microM) like nifedipine (NIF, 30 pM) caused a concentration-dependent displacement to the right of the dose-response curve to Bay K 8644 (0.1-100 nM) in K+-depolarized preparations. 6. Higher concentration of both FORS (1 and 3 microM) and NIF (0.01 and 0.03 nM) also reduced the maximal response of Bay K 8644. 7. Both drugs caused a concentration-dependent relaxation of uterine muscle contracted by Bay K 8644 (100 nM) and K+ (80 nM), but neither FORS nor NIF discriminated the two types of contraction.

Characterization and solubilization of the nitrendipine binding protein from canine small intestinal circular smooth muscle

The binding of nitrendipine, a calcium channel blocking agent, to the microsomes prepared from canine small intestinal circular smooth muscle was characterized. The binding of this 1,4-dihydropyridine to the membrane was reversible, saturable and of high affinity with a dissociation constant of 0.28 nM and a maximal binding of 91 fmol/mg microsomal protein. The binding occurred with an association rate constant of 0.094 nM-1 min-1 and dissociated at the rate of 0.0498 min-1. These rate constants gave a value of 0.52 nM for the dissociation constant of the binding. The binding was inhibited in a competitive fashion by other dihydropyridines (nifedipine, nimodipine, nisoldipine, PN200-110) with inhibition constants in the range of 0.1 to 1.0 nM. The binding was also inhibited by verapamil and D-600 but only at much higher concentrations. Diltiazem increased the nitrendipine binding to the membranes. The nitrendipine binding protein was solubilized using the detergent 3-[(3-cholamidopropyl)dimethyl-ammonio]-1-propanesulfonate (CHAPS). The solubilized material bound nitrendipine with a dissociation constant of 0.51 nM giving maximal binding of 70 fmol/mg protein. The binding of the solubilized material resembled the membrane bound material in inhibition by the dihydropyridines, verapamil and D-600 and in the increase by diltiazem. Thus we have solubilized the nitrendipine binding protein without changing its binding properties substantially.

Effect of diphenylhydantoin on drug and calcium induced contractions of isolated rat uterus: a comparative study with nifedipine

The sustained contractions induced by K+, acetylcholine (ACh) and oxytocin (Ot) were inhibited in a concentration-dependent fashion by diphenylhydantoin (DPH) and nifedipine (NIF) in the following order of sensitivity: K+ greater than ACh greater than Ot. Previous incubation of DPH and NIF produced a non-competitive antagonism towards ACh and Ot-induced contractions. Increasing of calcium (Ca2+) concentration (0.2-1.5 mM) completely reverse the inhibitory effect of DPH and NIF, suggesting a competitive type of antagonism, between Ca2+ and DPH. A clear difference between DPH and NIF actions was observed when the K+-depolarizing solution was used. In this condition, DPH caused a parallel and concentration-related rightward displacement of the dose-response curves of Ca2+ (pA2 = 4.91 +/- 0.1), while NIF produced a rightward displacement allied to a significant reduction of the Ca2+ maximal response. DPH, but not NIF, produced a concentration-dependent relaxation in sustained contraction induced by Ca2+ (1 mM) in depolarized tissue.