Calcium signaling cascades differentially regulate PGF2α-induced myometrial contractions in water buffaloes (Bubalus bubalis)

This study unravels the differential involvement of calcium signaling pathway(s) in PGF_2α-induced contractions in myometrium of nonpregnant (NP) and pregnant buffaloes. Compared to the myometrium of pregnant animals, myometrium of NP buffaloes was more sensitive to PGF_2α as manifested by changes in mean integral tension (MIT) and tonicity. In the presence of nifedipine, myometrial contraction to PGF_2α was significantly attenuated in both NP and pregnant uteri; however, mibefradil and NNC 55-0396 produced inhibitory effects only in uterus of pregnant animals, thus suggesting the role of extracellular Ca²⁺ influx through nifedipine-sensitive L-type Ca²⁺-channels both in NP and pregnant, but T-type Ca²⁺ channels seem to play a role only during pregnancy. Entry of extracellular Ca²⁺ is triggered by enhanced functional involvement of Pyr3-sensitive TRPC3 channels and Rho-kinase pathways as evidenced by a significant rightward shift of the concentration-response curve of PGF_2α in the presence of Pyr3 and Y-27632 in NP myometrium. But significant down-expressions of TRPC3 and Rho-A proteins during pregnancy apparently facilitate uterine quiescence. In the presence of Ca²⁺-free solution and cyclopiazonic acid (SERCA blocker), feeble contraction to PGF_2α was observed in both NP and pregnant myometrium which suggests minor role of intracellular source of Ca²⁺ in mediating PGF_2α-induced contractions in these tissues.

The functional and molecular studies on involvement of hydrogen sulphide in myometrial activity of non-pregnant buffaloes (Bubalus bubalis)

Background

Hydrogen sulphide (H₂S), a member of the gasotransmitters family, is known to play patho-physiological role in different body systems including during pregnancy. But its involvement in myometrial spontaneity and associated signalling pathways in uterus in non-pregnant animals is yet to be studied. Present study describes the effect of L-cysteine, an endogenous H₂S donor, on isolated myometrial strips of non-pregnant buffaloes and the underlying signaling mechanism(s).

Results

L-cysteine (10 nM-30 mM) produced concentration-dependent contractile effect on buffalo myometrium which was extracellular Ca²⁺ and L-type calcium channels-dependent. Significant rightward shift of dose-response curve of L-cysteine was observed with significant decrease in maxima in the presence of amino-oxyacetic acid (AOAA; 100 μM) and d, l-propargylglycine (PAG; 100 μM), the specific blockers of cystathionine β-synthase (CBS) and cystathionine γ-lyase (CSE), respectively. Existence of CBS enzyme of 63 kDa and CSE of 45 kDa molecular weights was confirmed by western blot using specific antibodies and also by immunohistochemistry.

Conclusions

Endogenous H₂S along with its biosynthetic enzymes (CBS and CSE) is evidently present in uteri of non-pregnant buffaloes and it regulates spontaneity in uteri of non-pregnant buffaloes and this effect is dependent on extracellular Ca²⁺ influx through nifedipine-sensitive L-type calcium channels. Thus H₂S-signalling pathway may be a potential target to alter the uterine activities in physiology and patho-physiolgical states.

ATP-sensitive and maxi potassium channels regulate BRL 37344-induced tocolysis in buffaloes-an in vitro study

Cellular coupling of beta₃-adrenoceptors (β₃-ADR) to potassium channels in myometrium is largely unknown. In vitro study was undertaken to unravel the presence of β₃-adrenergic receptors (ADR) and the role of K⁺-channels in mediating β₃-ADR-induced relaxation in isolated myometrial strips from cyclic non-pregnant water buffaloes. Isometric tension was recorded in isolated myometrial strips using data acquisition system based physiograph. Compared to SR 59230A, BRL 37344 was found to be more potent in inducing β₃-dependent myometrial relaxation which was significantly (p < 0.05) inhibited in the presence of β₃ antagonist, SAR 150640. The immunoreactive protein to β₃-ADR was also detected in membrane fraction of myometrial protein. Further, incubation with BRL 37344 (10 μM) significantly (p < 0.05) increased c-AMP accumulation (37.58 ± 9.52 pmol/mg protein; n = 4) in the myometrial strips compared to basal c-AMP level (16.85 ± 3.87 pmol/mg protein; n = 4). The concentration response curves (CRC) of BRL 37344 were significantly (p < 0.05) shifted towards right in the presence of K_ATP channels specific blocker, glibenclamide (10 μM) and maxi K⁺-channels (BK_Ca) specific blocker, iberiotoxin (100 nM), with decrease in both efficacy and potency as compared to control. However, 4-aminopyridine (4-AP), a specific blocker of the voltage gated K⁺-channels (Kv), failed to alter the CRC of BRL 37344. Existence of immunoreactive protein to Kir6.1, α-subunit of BK_Ca and Kv1.1 channels were also detected in the membrane fraction of myometrial protein. Based on the above findings, it can be concluded that BRL 37344 is a potent stimulator of β₃-adrenoceptors in buffalo myometrium and besides mediating their effect through rise in c-AMP, they are coupled to K_ATP and BK_Ca channels in inducing tocolytic effects.

WITHDRAWN: Differential involvement of L- and T-type Ca2+ channels, store-operated calcium channel (TRPC) and Rho-kinase signaling pathway(s) in PGF2α-induced contractions in myometrium of non-pregnant and pregnant buffaloes (Bubalus bubalis)

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Extra and intracellular calcium signaling pathway(s) differentially regulate histamine-induced myometrial contractions during early and mid-pregnancy stages in buffaloes (Bubalus bubalis)

This study examines the differential role of calcium signaling pathway(s) in histamine-induced uterotonic action during early and mid-pregnancy stages in buffaloes. Compared to mid pregnancy, tonic contraction, amplitude and mean-integral tension were significantly increased by histamine to produce myometrial contraction during early pregnancy with small effects on phasic contraction and frequency. Although uterotonic action of histamine during both stages of pregnancy is sensitive to nifedipine (a L-type Ca²⁺ channels blocker) and NNC55-0396 (T-type Ca²⁺ channels blocker), the role of extracellular calcium seems to be more significant during mid-pregnancy as in this stage histamine produced only 9.38±0.96% contraction in Ca²⁺ free-RLS compared to 21.60±1.45% in uteri of early pregnancy stage. Intracellular calcium plays major role in histamine-induced myometrial contraction during early pregnancy as compared to mid pregnancy, as in the presence of cyclopiazonic acid (CPA) Ca²⁺-free RLS, histamine produced significantly higher contraction in myometrial strips of early-pregancy in comparison to mid-pregnancy (10.59±1.58% and 3.13±0.46%, respectively). In the presence of U-73122, the DRC of histamine was significantly shifted towards right with decrease in maximal effect (E_max) only in early pregnancy suggesting the predominant role of phospholipase-C (PL-C) in this stage of pregnancy.

ATP-sensitive potassium channels modulate in vitro tocolytic effects of β₂-adrenergic receptor agonists on uterine muscle rings in rats in early but not in late pregnancy

Aim

To investigate whether ATP-sensitive potassium (K_ATP) channels modulate the tocolytic effect of β₂-AR agonists (ritodrine and salmeterol) in early-pregnant (day 6) and late-pregnant (day 22) rat uterus in vitro, in order to examine the relation between the K_ATP channel sulphonylurea-binding regulatory subunit (SUR) expression and pharmacological reactivity of β₂-AR agonists.

Methods

The tocolytic effects of ritodrine and salmeterol (10^-10-10^-5 M) on spontaneous rhythmic contractions were investigated cumulatively, alone, or in the presence of the K_ATP channel blocker glibenclamide (10^-6 M) and the K_ATP channel opener pinacidil (10^-9-10^-7 M) after 5-min preincubation.

Results

β₂-AR agonist induced myometrial relaxation was inhibited by glibenclamide and enhanced by pinacidil on day 6, when SUR1 expression levels were high. Neither glibenclamide nor pinacidil mediated tocolytic effect was measured on day 22.

Conclusion

Low expression of the K_ATP channels at the end of gestation may facilitate enhanced excitability and contractility in the rat myometrium. The combination of a betamimetic and a K_ATP channel opener will therefore not be of therapeutic relevance in the treatment of preterm delivery.

Calcium influx and release mechanism(s) in histamine-induced myometrial contraction in buffaloes

The present study was undertaken to characterize the presence of histamine H1R using molecular biology tools and unravel the influx and release mechanism(s) involved in calcium signalling cascades in histamine-induced myometrial contraction in buffaloes. The presence of H1R mRNA transcript and immunoreactive membrane protein in buffalo myometrium was confirmed by RT-PCR and Western blot analysis. Further, histamine produced concentration-dependent (1nM-10μM) contraction in buffalo myometrium with a potency of 7.13±0.11. When myometrial strips were pre-incubated either with Ca(2+) free solution or with nifedipine, a L-type Ca(2+) channel blocker, dose response curve (DRC) of histamine was significantly (P<0.05) shifted towards right with decline in maximal contraction (Emax). Reduction in Emax of histamine in the presence of nifedipine (55.75±3.10%) was significantly (P<0.001) greater than that in the presence of ruthenium red (93.61±3.43%), a blocker of IP3-gated and RyR-sensitive Ca(2+) channels. Moreover, histamine produced only 26.87±1.99% of the maximum contraction in the presence of both nifedipine and CPA (blocker of sarco-endoplasmic reticulum Ca(2+)-ATPase). Interestingly, following concurrent exposure to U-73122 (a PL-C inhibitor) and nifedipine, the DRC of histamine was significantly (P<0.05) shifted towards left with increase in maximal contraction (126.30±3.36%). Our findings in buffalo uterus thus suggest that influx of extracellular calcium plays a major role in histamine-induced myometrial contraction, while release of intracellular calcium through calcium-release channels of sarcoplasmic reticulum has a minor role. A possible involvement of non-selective cation channels in histamine-induced myometrial contraction cannot be ruled out, and therefore requires further investigations.