The testicular capsule contracts in response to noradrenaline and adrenaline, but the effects of adrenoceptor agonists, as for instance clonidine, had not yet been thoroughly evaluated. The testicular capsule from adult male Wistar rats was isolated and mounted in organ bath and cumulative concentration curves were performed for clonidine and other adrenergic agonists in the absence or presence of α-adrenoceptors antagonists. The order of potency for agonists (pD2) was clonidine=adrenaline>UK 14,304>noradrenaline>phenylephrine>methoxamine. The consecutive curves for clonidine showed desensitization with 3-fold rightward shift and Emax reduction of 40%. The noradrenaline curves were 4.5, 19 and 190-fold less potent after clonidine pretreatment at 10−5, 10−4 or 10−3 M for 10 min, respectively, added to Emax decrease by about 20%. Clonidine (10−5 M for 10 min) was unable to alter the noradrenaline curves if the treatment was made in the presence of idazoxan (α2-adrenoceptor antagonist) whereas prazosin (α1-adrenoceptor antagonist) was ineffective. The effect of idazoxan 3×10−7 M on noradrenaline curves was decreased by 50% after clonidine pretreatment, as reflected by the concentration ratio of 5.2±1.2 (treated tissue) and 10.1±1.0 (untreated tissue). However, the concentration ratio for prazosin 3×10−8 M was unchanged. After phenoxybenzamine (irreversible antagonist of α1-adrenoceptor) pretreatment, the residual noradrenaline contraction was antagonized by idazoxan or prazosin with pKB values of 7.8 and 5.1, respectively. The results indicate the presence of α2-adrenoceptors in testicular capsule. Furthermore, these receptors may be desensitized by clonidine, causing a decreased potency of noradrenaline.

In the normotensive rat atrium, adenosine-5'-triphosphate and uridine-5'-triphosphate exert a biphasic effect consisting of an initial negative inotropic effect (NIE) followed by a subsequent positive inotropic effect (PIE). We comparatively studied these responses in normotensive Wistar rats (NWRs) and spontaneously hypertensive rats (SHRs). Compared with NWRs, the NIE responses in the atria were lower and the PIE responses were higher in SHRs. The P1 purinoceptor antagonist, D 8-cyclopentyl-1,3-dipropylxanthine, partially blocked the NIE responses of both ATP and UTP and mildly enhanced the PIE responses in both NWRs and SHRs. Furthermore, the P2 purinoceptor blockers suramin and pyridoxalphosphate-6-azophenyl-2',4'-disulphonic acid tetrasodium salt induced a pronounced block of the PIE responses in both atria types. The PIE responses to ATP were inhibited more efficiently by nifedipine. These responses were depressed by ryanodine and, to a lesser extent, carbonyl cyanide 3-chlorophenylhydrazone in SHR atria compared with NWR atria. The higher responses in SHR rats suggest the existence of an augmented endoplasmic reticulum Ca(2+) store and faster mitochondrial Ca(2+) cycling in SHR atria compared with NWR atria. These data support the hypothesis that a dysfunction of purinergic neurotransmission and enhanced sympathetic activity are contributing factors in the pathogenesis of hypertension.

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Autonomic nerves release ATP, which is processed into adenosine in the synaptic cleft. Adenosine and ATP exert a negative chronotropic effect in the heart. This study aims to evaluate adenosine and P2 receptors and cellular signalling in cardiac arrest produced by purines in the heart. Right atria of adult Wistar rats were used to evaluate the effects of adenosine, ATP and CPA (an adenosine A1 receptor agonist), in the presence and absence of DPCPX, an adenosine A1 receptor antagonist. Effects of adenosine A2 and A3 receptors agonists and antagonists were also investigated. Finally, involvement of calcium and potassium channels in these responses was assessed using BayK 8644 and 4-Aminopyridine. Cumulative concentration-effect curves of adenosine and CPA resulted in a negative chronotropic effect culminating in cardiac arrest at 1000μM (adenosine) and 1µM (CPA). Furthermore, ATP produced a negative chronotropic effect at 1-300µM and cardiac arrest at 1000μM in the right atrium. ATPγS (a non-hydrolysable analogue of ATP) reduced chronotropism only. The effects of adenosine, CPA and ATP were inhibited by DPCPX, a selective adenosine A1 receptor antagonist. The selective adenosine A2 and A3 receptors antagonists did not alter the chronotropic response of adenosine. 4-Aminopyridine, a blocker of potassium channels at 10mM, prevented the cardiac arrest produced by adenosine and ATP, while BayK 8644, activator of calcium channels, did not prevent cardiac arrest. Adenosine A1 receptor activation by adenosine and ATP produces cardiac arrest in the right atrium of Wistar rats predominantly through activation of potassium channels.

SUMMARYThe use of clonidine, a selective agonist of a 2 -adrenoceptors, is related to the fertility impairment. Thus, it has been described that changes in the epididymal function are related to the loss of fertility. Therefore, this study was sought to further evaluate the effects of clonidine in the rat distal cauda epididymis contractions and its consequence in the sperm parameters. The in vitro effects of clonidine in the isolated distal cauda epididymis were evaluated by pharmacological experiments. The consecutive contractile responses for clonidine in distal cauda epididymis showed desensitization. The noradrenaline-induced contractions were desensitized after in vitro clonidine pre-treatment (10 À5 M for 10 min). Clonidine was unable to alter the noradrenaline contractions if the in vitro pre-treatment was made in the presence of idazoxan (a 2 -adrenoceptor antagonist), whereas prazosin (a 1 -adrenoceptor antagonist) was ineffective. Moreover, the in vitro clonidine pre-treatment increased frequency and amplitude of spontaneous contraction of distal cauda epididymis. In addition, to induce in vivo desensitization of a 2 -adrenoceptors, male Wistar rats were treated with crescent doses of clonidine and distal cauda of epididymis contraction and sperm parameters were analyzed. The in vivo treatment with clonidine diminished the potency of the contractions induced by adrenergic agonists and augmented the frequency and amplitude of spontaneous contraction of distal cauda epididymis. This treatment also altered the sperm transit time in epididymis, epididymal sperm reserves, sperm lipid peroxidation, and antioxidant enzymes activity. The results suggest that clonidine was able to affect the sperm quantity and quality by decreasing the transit time related to the increase in the frequency and amplitude of spontaneous contractions in epididymis, although the contractions induced by adrenergic agonists were desensitized.