Forskolin inhibition of K+ current in pregnant rat uterine smooth muscle cells

Relaxant and anti-inflammatory effect of two thalidomide analogs as PDE-4 inhibitors in pregnant rat uterus

The aim of this study was to evaluate the relaxant and anti-inflammatory effects of two thalidomide analogs as phosphodiesterase-4 (PDE-4) inhibitors in pregnant rat uterus. Uteri from Wistar female rats were isolated at 19 day of pregnancy. Uterine samples were used in functional studies to evaluate the inhibitory effects of the thalidomide analogs, methyl 3-(4-nitrophthalimido)-3-(3,4-dimethoxyphenyl)-propanoate (4NO2PDPMe) and methyl 3-(4-aminophthalimido)-3-(3,4-dimethoxyphenyl)-propanoate (4APDPMe), on prostaglandin-F2α (PGF2α)-induced phasic, K⁺-induced tonic, and Ca²⁺-induced contractions. Accumulation of cAMP was quantified in uterine homogenates by ELISA. Anti-inflammatory effect was assessed by using ELISA for determination of the pro-inflammatory cytokines tumor necrosis factor-α (TNFα) and interleukin (IL)-1β, and anti-inflammatory IL-10, from uterine explants stimulated with lipopolysaccharide (LPS). Nifedipine, forskolin and rolipram were used as positive controls where required. Both thalidomide analogs induced a significant inhibition of the uterine contractions induced by the pharmaco- and electro-mechanic stimuli. Nifedipine and forskolin were more potent than the analogs to inhibit the uterine contractility, but these were more potent than rolipram, and 4APDPMe was equieffective to nifedipine. Thalidomide analogs increased uterine cAMP-levels in a concentration-dependent manner. The LPS-induced TNFα and IL-1β uterine secretion was diminished in a concentration-dependent fashion by both analogs, whereas IL-10 secretion was increased significantly. The thalidomide analogs induced utero-relaxant and anti-inflammatory effects, which were associated with the increased cAMP levels as PDE-4 inhibitors in the pregnant rat uterus. Such properties place these thalidomide analogs as potentially safe and effective tocolytic agents in a field that urgently needs improved pharmacological treatments, as in cases of preterm labor.

Differential expression of Kv4 pore-forming and KChIP auxiliary subunits in rat uterus during pregnancy

Regulation of voltage-gated K(+) (K(v)) channel expression may be involved in controlling contractility of uterine smooth muscle cells during pregnancy. Functional expression of these channels is not only controlled by the levels of pore-forming subunits, but requires their association with auxiliary subunits. Specifically, rapidly inactivating K(v) current is prominent in myometrial cells and may be carried by complexes consisting of Kv4 pore-forming and KChIP auxiliary subunits. To determine the molecular identity of the channel complexes and their changes during pregnancy, we examined the expression and localization of these subunits in rat uterus. RT-PCR analysis revealed that rat uterus expressed all three Kv4 pore-forming subunits and KChIP2 and -4 auxiliary subunits. The expression of mRNAs for these subunits was dynamically and region selectively regulated during pregnancy. In the corpus, Kv4.2 mRNA level increased before parturition, whereas the expression of Kv4.1 and Kv4.3 mRNAs decreased during pregnancy. A marked increase in KChIP2 mRNA level was also seen at late gestation. In the cervix, the expression of all three pore-forming and two auxiliary subunit mRNAs increased at late gestation. Immunoprecipitation followed by immunoblot analysis indicated that Kv4.2-KChIP2 complexes were significant in uterus at late pregnancy. Kv4.2- and KChIP2-immunoreactive proteins were present in both circular and longitudinal myometrial cells. Finally, Kv4.2 and KChIP2 mRNA levels were similarly elevated in pregnant and nonpregnant corpora of one side-conceived rats. These results suggest that diffusible factors coordinate the pregnancy-associated changes in molecular compositions of myometrial Kv4-KChIP channel complexes.

Calcium signaling mediated by P2Y receptors in mouse taste cells

Evidence implicates a number of neuroactive substances and their receptors in mediating complex cell-to-cell communications in the taste bud. Recently, we found that ATP, a ubiquitous neurotransmitter/neuromodulator, mobilizes intracellular Ca2+ in taste cells by activating P2Y receptors. Here, P2Y receptor-cellular response coupling was characterized in detail using single cell ratio photometry and the inhibitory analysis. The sequence of underlying events was shown to include ATP-dependent activation of PLC, IP3 production, and IP3 receptor-mediated Ca2+ release followed by Ca2+ influx. Data obtained favor SOC channels rather than receptor-operated channels as a pathway for Ca2+ influx that accompanies Ca2+ release. Intracellular Ca2+ mobilized by ATP is apparently extruded by the plasma membrane Ca2+-ATPase, while a contribution of the Na+/Ca2+ exchange and other mechanisms of Ca2+ clearance is negligible. Cyclic AMP-dependent phosphorylation is likely to control a gain of the phosphoinositide cascade involved in ATP transduction. ATP-responsive taste cells are abundant in circumvallate, foliate, and fungiform papillae. Taken together, our observations point to a putative role for ATP as a neurotransmitter operative in the taste bud.

New developments in the management of preterm labor

Current management of preterm labor has not changed the incidence of preterm delivery; therefore, significant research effort has been concentrated on the search for new methods of management. New tocolytics like inhibitors of cyclooxygenase 2 and nitric oxide donors have been tested in animal models and in preliminary clinical trials with promising results. Inhibition of cervical ripening may be one alternative to tocolysis. This new approach has a potential to be a valuable method of management of preterm labor if human studies confirm the promising results reported in animals. Growing evidence suggests that premature delivery may be associated with infection or fetal growth abnormalities, with dire consequences to the fetus. If these associations are to be included in risk and benefit assessment, then inhibition of preterm labor may prove to be detrimental to the fetus.

Adenylate cyclase and potassium channels are involved in forskolin- and 1,9-dideoxyforskolin-induced inhibition of pregnant rat uterus contractility

OBJECTIVE

We sought to study the contribution of potassium channels in the effect of forskolin and 1,9-dideoxyforskolin on uterine contractility in the pregnant rat.

STUDY DESIGN

Rings taken from the middle portions of uterine horns from rats at 16 days of gestation were positioned in organ chambers containing physiologic salt solution bubbled with 5% carbon dioxide in air (37 degrees C, pH approximately 7.4) for isometric tension recording under 2 g passive tension. The effects of cumulative concentrations of forskolin and 1,9-dideoxyforskolin in the absence or presence of an adenylate cyclase inhibitor (MDL-12,330A, 10(-5) mol/L), a nonselective potassium channel blocker (tetrabutylammonium, 10(-4) mol/L), or an adenosine triphosphate-dependent potassium channel blocker (glibenclamide 10(-5) mol/L) were studied.

RESULTS

Both forskolin and, to a lesser extent, 1,9-dideoxyforskolin inhibit uterine contractions. Tetrabutylammonium, glibenclamide, and MDL-12, 330A attenuated the effects of forskolin, whereas glibenclamide was less effective against 1,9-dideoxyforskolin.

CONCLUSION

Activation of adenylate cyclases, as well as adenosine triphosphate-dependent potassium channels and, to a greater extent, calcium-dependent potassium channels, is involved in the inhibitory effect of forskolin in uterine rings from rats at 16 days of gestation. Inhibition of uterine contractions by 1,9-dideoxyforskolin is less than that by forskolin and involves activation of adenylate cyclase and calcium-dependent potassium channels. Whether activation of guanylate cyclase is involved in the effect of the agents on calcium-dependent potassium channels needs further investigation. 1, 9-Dideoxyforskolin is not an inactive isomer of forskolin in rat uterine rings.

Potassium currents in freshly dissociated uterine myocytes from nonpregnant and late-pregnant rats

In freshly dissociated uterine myocytes, the outward current is carried by K+ through channels highly selective for K+. Typically, nonpregnant myocytes have rather noisy K+ currents; half of them also have a fast-inactivating transient outward current (ITO). In contrast, the current records are not noisy in late pregnant myocytes, and ITO densities are low. The whole-cell IK of nonpregnant myocytes respond strongly to changes in [Ca2+]o or changes in [Ca2+]i caused by photolysis of caged Ca2+ compounds, nitr 5 or DM-nitrophene, but that of late-pregnant myocytes respond weakly or not at all. The Ca2+ insensitivity of the latter is present before any exposure to dissociating enzymes. By holding at -80, -40, or 0 mV and digital subtractions, the whole-cell IK of each type of myocyte can be separated into one noninactivating and two inactivating components with half-inactivation at approximately -61 and -22 mV. The noninactivating components, which consist mainly of iberiotoxin-susceptible large-conductance Ca2+-activated K+ currents, are half-activated at 39 mV in nonpregnant myocytes, but at 63 mV in late-pregnant myocytes. In detached membrane patches from the latter, identified 139 pS, Ca2+-sensitive K+ channels also have a half-open probability at 68 mV, and are less sensitive to Ca2+ than similar channels in taenia coli myocytes. Ca2+-activated K+ currents, susceptible to tetraethylammonium, charybdotoxin, and iberiotoxin contribute 30-35% of the total IK in nonpregnant myocytes, but <20% in late-pregnant myocytes. Dendrotoxin-susceptible, small-conductance delayed rectifier currents are not seen in nonpregnant myocytes, but contribute approximately 20% of total IK in late-pregnant myocytes. Thus, in late-pregnancy, myometrial excitability is increased by changes in K+ currents that include a suppression of the ITO, a redistribution of IK expression from large-conductance Ca2+-activated channels to smaller-conductance delayed rectifier channels, a lowered Ca2+ sensitivity, and a positive shift of the activation of some large-conductance Ca2+-activated channels.

Enhancement of K+ currents by stimulation of protein kinase C in the uterine smooth muscle cells of the pregnant rat

1. Effects of phorbol esters on the K+ currents in isolated rat uterine smooth muscle cells during (18-day) pregnancy were examined using whole-cell voltage-clamp modes. All experiments were performed at room temperature. 2. Test pulses were applied between -20 to + 90 mV from a holding potential of -40 mV. Initially, a transient outward current (ITO) was activated, and outward K+ current (IK) was followed. Threshold potential was - 10 to 0 mV, and the activation was voltage-dependent. At - 80 mV, ITO and IK were 17.8 +/- 3.3 and 13.2 +/- 2.6 pA/pF as a current density. Membrane capacitance was 64.0 +/- 11.5 pF (n = 8). 3. At 0.1 microM, 12-O-tetradecanoylphorbol-13-acetate (TPA) and 4-beta-phorbol-12-13-dibutyrate (PDB) enhanced IK at +80mV by 14.5 +/- 2.0% (n = 8, P < 0.05) and 23.5 +/- 2.2% (n = 7, P < 0.01). Also, ITO at +80mV was increased by 22.1 +/- 2.1% (n = 8, P < 0.01) at 1 microM TPA and by 22.7 +/- 3.0 (n = 7, P < 0.05) at 0.1 microM PDB, significantly. 4. These results indicate that the IK and ITO currents are present in the uterine smooth muscle cells of pregnant rat, and PK-C stimulation modulates the K+ currents, resulting in the regulation of physiological contraction of the uterine muscle during pregnancy.

Role of cyclic AMP and protein kinase A in K+ channel activation by calcitonin gene-related peptide (CGRP) in the guinea-pig ureter

1. The aim of this study was to assess whether agents that interfere with the intracellular actions of cAMP and activation of protein kinase A (PKA) prevent the inhibitory action of human alpha-calcitonin gene-related peptide (CGRP) in the guinea-pig ureter smooth muscle. The action of CGRP was compared to that of the K+ channel opener, cromakalim, and the adenylyl cyclase activator, forskolin, toward electrical field stimulation- (EFS) induced myogenic twitch contractions of the ureter. To further verify the role of cAMP in the action of CGRP, we also studied the effect of stable cAMP analogues and of the phosphodiesterase inhibitor, 3-isobutyl-1-methylxanthine (IBMX). 2. Maximally effective concentrations of CGRP (0.1 microM) or forskolin (10 microM) produced a transient suppression of twitches. Cromakalim (3 microM) likewise produced a prompt suppression of twitches that in most cases exceeded 15 min. The early suppressant effect of CGRP or forskolin was inhibited by 1 or 10 microM glibenclamide; about 30% of the effect of CGRP was glibenclamide-resistant. The effect of cromakalim was totally suppressed by glibenclamide. 3. The inhibitory effect of CGRP was concentration-dependently reduced by low concentrations of barium chloride (IC50 63 microM), which blocked with similar potency the inhibitory action of cromakalim (IC50 60 microM). Glibenclamide (10 nM-10 microM) concentration-dependently inhibited the effect of CGRP and cromakalim with IC50S of 0.13 and 0.72 microM, respectively. 4. The cAMP analogues dibutyrye-cAMP (1-3 mM), 8-(4-chlorophenylthio)cAMP (0.3-1 mM) and Sp-cAMP monophosphothioate (0.1-0.3 mM) were either ineffective or poorly effective in inhibiting twitches. The cGMP analog, 8Br-cGMP (100-300 microM) produced a slowly developing, glibenclamide (1 microM)-resistant partial inhibition (25-30%) of twitches. 5. IBMX (1-300 microM) produced a concentration-dependent inhibition of twitches (EC50 16 microM). IBMX (100 microM) produced a large (peak 91%) and transient inhibition: glibenclamide (1 microM) blocked the early peak of the inhibitory action of IBMX, similar to the effect observed toward CGRP and forskolin.

Potassium currents in rat colonic smooth muscle cells and changes during development and aging

In a previous study on freshly isolated single smooth muscle cells from the circular layer of the rat distal colon, we reported that the L-type Ca2+ current density increased during development and gradually declined with further aging [ZI Xiong, N Sperelakis, N Noffsinger, C Fenoglio-Preiser (1993) Am J Physiol 265: C617-C625]. Since K+ current plays a key role in controlling excitability of the cells and hence the motility of the colon, in the present study the voltage-gated K+ channel currents, (IK) were investigated using the whole-cell voltage-clamp technique in colonic myocytes from rats of different ages. A Ca(2+)-sensitive K+ current [IK(Ca)] and two kinds of Ca(2+)-insensitive outward K+ currents were identified and characterized. IK(Ca) was recorded at potentials more positive than -40 mV in Ca(2+)-containing bath solution, and was blocked by Ca2+ channel antagonists and tetraethylammonium ion (TEA+). After removing Ca2+ from the bath solution and using a high ethylenebis(oxonitrilo)tetraacetate (EGTA, 4 mM) concentration in the pipette, two types of Ca(2+)-insensitive IK were recorded. The first and faster component was usually activated at potentials more positive than -50 mV, and was more sensitive to 4-aminopyridine (4-AP). In contrast, the second and slower (delayed) component was activated at potentials more positive than -30 mV, and was more sensitive to TEA. The total density of the Ca(2+)-insensitive IK component decreased dramatically during the neonatal period: from 32.2 +/- 3.2 pA/pF in 3-day-old rats to 17.8 +/- 2.6 pA/pF in 40-day-old rats; there was no further decline during aging (up to 480 days).(ABSTRACT TRUNCATED AT 250 WORDS)

Ion channels and the control of myometrial electrical activity

Understanding the role of ion channels in the generation of slow waves and action potentials in the myometrium is critical in designing strategies to regulate uterine contractile activity. The development of the patch clamp technique has allowed the identification of specific types of channels in the myometrium and provided insights into their regulation by hormones and drugs. Specifically, new studies suggest that KATP and KCa channel openers could be important tools in the management of inappropriate uterine contractions, but peripheral effects will have to be controlled. Conversely, blockers of these same channels may have some effects on dystocia. The study of contractant-operated channels in the myometrium is still in its infancy, but promises new insights into possible modes of regulation as well. Myometrial activity is controlled at a number of levels. The regulation of ion channels is an important aspect, but receptor-mediated actions that do not appear to be voltage- or ion-dependent presumably are also important contributors and hence are sites of potential modulation as well. Clearly, future multifaceted approaches to tocolysis, and perhaps also dystocia, may well include agents targeting the activity of ion channels.