A1- and A2-purinoceptors in the guinea-pig uterus

Modulation of Pb(II) caused aortal constriction by eugenol and carvacrol

Exposure to lead is known to cause vasoconstriction, exact mechanism of which remains to be elucidated. In this study, we investigate contractile responses of rat aortal rings equilibrated with Pb(II) in organ bath system, explore pathways responsible for hypercontraction and examine two ameliorators of lead-induced hypercontraction. At 1 μmol L(-1) Pb(II), aortal rings showed an average increase of 50% in isometric contraction. Incubation of rings, unexposed to Pb(II), with 1 μmol L(-1) sodium nitroprusside (nitric oxide (NO) donor), 100 μmol L(-1) apocynin (reactive oxygen species (ROS) inhibitor), and 100 μmol L(-1) indomethacin (cyclooxygenase inhibitor) lead to decrease in phenylephrine-induced contraction by 31, 27, and 29%, respectively. This decrease of contraction for Pb(II)-exposed rings was 48, 53, and 38%, respectively, indicating that ROS- and NO-dependent components of contractions are significantly elevated in Pb(II)-induced hypercontraction. Cyclooxygenase-dependent contractile component did not show significant elevation. Eugenol and carvacrol are plant-derived phenols known to possess antioxidant activity and hence could act as possible ameliorators of hypercontraction. At saturating concentrations of 100 μmol L(-1), eugenol and carvacrol caused a decrease in contraction by 38 and 42% in unexposed rings and 46 and 50% in Pb(II)-exposed rings. Co-incubation of rings with eugenol/carvacrol and various inhibitors suggests that both these active principles exert their relaxant effect via quenching of ROS and stimulation of NO synthesis. To conclude, Pb(II) is shown to induce hypercontraction of aortal rings through elevation of ROS and depletion of NO. This hypercontraction is effectively mitigated by eugenol and carvacrol.

Regulation of smooth muscle contraction by the epithelium: role of prostaglandins

As an analog to the endothelium situated next to the vascular smooth muscle, the epithelium is emerging as an important regulator of smooth muscle contraction in many vital organs/tissues by interacting with other cell types and releasing epithelium-derived factors, among which prostaglandins have been demonstrated to play a versatile role in governing smooth muscle contraction essential to the physiological and pathophysiological processes in a wide range of organ systems.

Evidence of direct smooth muscle relaxant effects of the fibrate gemfibrozil

Fibrates are commonly employed to treat abnormal lipid metabolism via their unique ability to stimulate peroxisome proliferator-activated receptor alpha (PPARalpha). Interestingly, they also decrease systemic arterial pressure, despite recent evidence that PPAR alpha may contribute to expression of renin and related hypertension. Yet, mechanisms responsible for their potential antihypertensive activity remain unresolved. Rapid decreases in arterial pressure following bolus intravenous injections of bezafibrate strongly suggest they may relax arterial smooth muscle directly. But since bezafibrate is highly susceptible to photodegradation in aqueous media, it has never been critically tested for this possibility in vitro with isolated arterial smooth muscle preparations. Accordingly, we tested gemfibrozil which is resistant to photodegradation. We examined it over a therapeutically-relevant range (50-400 microM) for both acute and delayed relaxant effects on contractions of the isolated rat tail artery; contractions induced by either depolarizing its smooth muscle cell membranes with high potassium or stimulating its membrane-bound receptors with norepinephrine and arginine-vasopressin. We also examined these same gemfibrozil levels for effects on spontaneously-occurring phasic rhythmic contractile activity, typically not seen in arteries under in vitro conditions but commonly exhibited by smooth muscle of uterus, duodenum and bladder. We found that gemfibrozil significantly relaxed all induced forms of contraction in the rat tail artery, acutely at the higher test levels and after a delay of a few hours at the lower test levels. The highest test level of gemfibrozil (400 microM) also completely abolished spontaneously-occurring contractile activity of the isolated uterus and duodenum and markedly suppressed it in the bladder. This is the first evidence that a fibrate drug can directly relax smooth muscle contractions, either induced by various contractile agents or spontaneously-occurring. These findings are particularly relevant to both the recently renewed concern over the impact of fibrates on hypertension and a new understanding of their gastrointestinal side effects.

The birth and postnatal development of purinergic signalling

The purinergic signalling system is one of the most ancient and arguably the most widespread intercellular signalling system in living tissues. In this review we present a detailed account of the early developments and current status of purinergic signalling. We summarize the current knowledge on purinoceptors, their distribution and role in signal transduction in various tissues in physiological and pathophysiological conditions.

Evidence for the presence of both P1 and P2 purinoceptors in the rat myometrium

1. Adenosine, adenosine triphosphate (ATP) and some stable analogues of adenosine inhibited field stimulation-induced contractions of the uterus from rats treated with oestradiol cypionate (20 micrograms/kg, s.c.) 1 day previously. Adenosine was twice as potent as ATP; both were potentiated by dipyridamole (10 mumol/L). 2. The order of agonist potency of adenosine and its analogues was: 5'-N-ethylcarboxamidoadenosine (NECA) > N6-cyclohexyladenosine > or = R-phenylisopropyladenosine = S-phenylisopropyladenosine = 2-chloroadenosine > or = adenosine > or = ATP > > 2-p-(2-carboxyethyl) phenethylamino-5'-N-ethylcarboxamidoadenosine. This order suggests the presence of P1 purinoceptors of the A2B subtype. 3. Responses to agonists were antagonized to differing extents by the P1 purinoceptor antagonist 8-phenyltheophylline (10 mumol/L). 4. In uterine preparations from rats pretreated for 2 days with oestrogen (20 micrograms/kg, s.c.) and for 1 day with progesterone (3 mg/animal, s.c.), the inhibitory potencies of adenosine and NECA were reduced, indicating hormonal regulation of uterine responsiveness to P1 purinoceptor agonists. 5. Stable analogues of ATP caused contractions of unstimulated myometrial preparations from oestrogen-treated animals, indicating activation of a P2 purinoceptor, possibly of the P2X subtype, because of the relative order of potency was alpha, beta-methylene ATP > beta, gamma-methylene ATP = ATP = 2-methylthio ATP.