Membrane potential responses to ionophoretically applied alpha-adrenoceptor agonists in the mouse anococcygeus muscle

Biology of the anococcygeus muscle

The anococcygeus is a smooth muscle tissue of the urogenital tract which, in the male, runs on to form the retractor penis. The motor innervation is classically sympathetic with noradrenaline as transmitter, but the relaxant parasympathetic transmitter has only recently been identified as nitric oxide. Indeed, the anococcygeus has provided an extremely useful model with which to probe the mechanisms underlying this novel nitrergic system, including the importance of physiological antioxidants in maintaining the potency of nitric oxide as a neurotransmitter. The cellular mechanisms of contraction and relaxation are slowly being clarified, with particular interest in the contribution of capacitative calcium entry and the guanylyl cyclase/cyclic GMP system. Many questions remain unanswered, however, including the precise physiological role of the muscle, the identity of substances released from subcellular vesicles of nitrergic nerves, the unusual sensitivity of the tissue to certain peptides (oxytocin and urotensin II), and the nature of store-operated channels through which calcium enters the cell to maintain contraction.

Electrophysiology of neuromuscular transmission in guinea-pig mesenteric veins

1. Neuromuscular transmission in the smooth muscle of mesenteric veins has been investigated by recording intracellular potential changes resulting from stimulation of the sympathetic nerves and comparing these potential changes with responses obtained by ionophoresis of noradrenaline. 2. Neural stimulation or exogenous noradrenaline acted similarly to cause two excitatory depolarizations, a slow response reported previously (Suzuki, 1981) and a separate fast depolarization. 3. The fast depolarization was distinct from the slow depolarizing response in that it had a different dependence on the level of stimulation, was readily desensitized and was more suppressed in low-chloride solution. 4. The fast but not the slow depolarization shared certain characteristics with constriction. The fast depolarization and constriction both increased with the intensity of stimulation; inactivation in both was dependent on the recovery interval between trains of stimuli and both were suppressed to a similar degree by antagonists to alpha-adrenoceptors. The fast depolarization was, however, not a prerequisite for constriction to occur. 5. The fast and slow depolarizations were activated after a long latency which had a high temperature coefficient consistent with the postulate that these responses are rate limited by intracellular biochemical reactions. 6. The fast depolarization was preferentially suppressed by prazosin, an antagonist to the alpha 1-adrenoceptor subtype. Suppression of the slow depolarization required relatively higher concentrations of antagonist, indicating that these responses were mediated by receptor interactions involving a different alpha-adrenoceptor subtype. 7. It is concluded that neuromuscular transmission in mesenteric veins occurs through activation of alpha-adrenoceptors. A number of responses result, including voltage-independent constriction and two distinct excitatory depolarizations which can lead to voltage-dependent constriction.

Agonist profile of ergometrine (ergonovine) on a population of postsynaptic alpha-adrenoceptors

Ergometrine (0.02-5 microM) produced concentration-related contractions of the mouse anococcygeus muscle, which were unaffected by cocaine (2 microM) or by pretreatment of mice with 6-hydroxydopamine. Contractions were reduced by alpha-adrenoceptor antagonists; the rank order of potency was prazosin greater than phentolamine greater than yohimbine. With phenoxybenzamine as antagonist, the estimated dissociation constant (KD) for ergometrine was 0.41 microM. It is concluded that ergometrine causes direct activation of postsynaptic alpha 1-adrenoceptors, and it is suggested that it acts on the same subtype of the receptor as imidazoline agonists.

The action of noradrenaline on single smooth muscle cells freshly dispersed from the guinea-pig pulmonary artery

Responses to ionophoretically-applied noradrenaline were investigated with micro-electrodes in whole tissue preparations and with patch pipettes in isolated cells dispersed from the guinea-pig pulmonary artery. In whole tissue and dispersed cells noradrenaline evoked monophasic depolarizations which had a similar time course. In dispersed cells the amplitude of electronic potentials was reduced during the noradrenaline-evoked depolarization. Under voltage clamp noradrenaline elicited an inward current, which persisted in 18 mM external potassium with the membrane potential set at the potassium equilibrium potential. In voltage clamp experiments the amplitude of current steps to hyperpolarizing voltage jumps was increased during the noradrenaline-induced inward current. These data suggest that the depolarization to noradrenaline in the guinea-pig pulmonary artery is mediated by an increase in membrane conductance.

Evidence for two mechanisms of depolarization associated with alpha 1-adrenoceptor activation in the rat anococcygeus muscle

Membrane potential responses in the rat isolated anococcygeus to bath-applied noradrenaline and field stimulation have been investigated by use of intracellular microelectrode and combined extracellular electrical and mechanical (sucrose gap) recording techniques. Intracellular recordings were made usually from tissues immobilized with hypertonic Krebs solution. Bath-application of noradrenaline produced depolarizations which consisted of two components; an initial 'fast' phase which peaked within 1-2 s and which was followed by a 'slow' sustained depolarization. Both components were concentration-dependent. Noradrenaline could also evoke oscillations in membrane potential which, unlike the 'fast' component of depolarization, were prevented by conditioning hyperpolarization of the membrane and were evoked by direct membrane depolarization with externally applied current pulses. Thus, the oscillations are voltage-dependent phenomena. Replacement of the external NaCl of the Krebs solution with an equimolar amount of Na benzenesulphonate abolished the noradrenaline-evoked 'fast' depolarization while the 'slow' phase was unaffected. This suggests that two mechanisms of depolarization are activated in this muscle by the bath-application of noradrenaline. The adrenergic excitatory junction potential was also abolished in Na benzenesulphonate. Prazosin reduced both the 'fast' and 'slow' components of depolarization produced by noradrenaline indicating their mediation by alpha 1-adrenoceptors. The membrane potential (-29 mV) at the maximum amplitude of the 'fast' depolarization was similar to the equilibrium potential (-27 mV) for the depolarization evoked by ionophoretically applied noradrenaline and which was obtained by extrapolation from the relationship between amplitude of the ionophoretic response and membrane potential displacement in the partition chamber. These results suggest that the 'fast' depolarization and the ionophoretic response are due to an increased membrane conductance, possibly to chloride.

Electrophysiological analysis of the nature of adrenoceptors in the rat basilar artery during development

The nature of adrenoceptors in basilar arteries of neonatal rats was investigated by means of electrophysiological techniques. In immature (2-6 day postnatal) rats, micro-injection of noradrenaline elicited a depolarization which consisted of two components. The initial 'fast' component (time to peak of 0.3-4s) was slightly reduced by phentolamine and was not antagonized by propranolol. The second 'slow' component (time to peak of about 50s) was not blocked by phentolamine but was antagonized by low concentrations (10(-7) M) of propranolol. In immature rats, micro-injection of isoprenaline was more potent than noradrenaline in evoking the 'slow' depolarization but less effective in eliciting the 'fast' response. The pharmacology with respect to adrenoceptor antagonists of both components of the isoprenaline- and noradrenaline-induced depolarizations was similar. There was some evidence of inhibitory beta-adrenoceptors in immature rat basilar vessels. In adult rats (6 week old) noradrenaline produced a large 'fast' depolarization which was followed by a 'slow' tail response. Both components were not antagonized by phentolamine or propranolol. It appears that in the basilar artery of neonatal rats there are excitatory alpha- and inhibitory beta-adrenoceptors but the major responses to noradrenaline and isoprenaline are mediated by gamma- and excitatory beta-receptors. In adult animals the gamma-adrenoceptor predominates. Experiments were carried out in which agonists were applied by ionophoresis. These results confirm the presence of excitatory beta-receptors in neonatal basilar vessels and show the response has slow kinetics and it is likely that the beta-receptors are distributed uniformly over the smooth muscle surface. In adult animals it was not possible to elicit an excitatory beta-receptor-mediated response. The ionophoretic application of noradrenaline never evoked a perceptible depolarization which could be attributed to gamma-adrenoceptor stimulation. This result is discussed in terms of receptor distribution with respect to synaptic function in a syncytium.

The effect of chloride removal on the responses of the isolated rat anococcygeus muscle to alpha 1-adrenoceptor stimulation

The effect of Cl removal on the depolarizations and contractions produced by activation of the alpha 1-adrenoceptor in the isolated rat anococcygeus was studied. In normal Krebs solution the ionophoretic application of noradrenaline and phenylephrine elicited large depolarizations; when NaCl was replaced with Na benzenesulphonate application of the agonists usually evoked no membrane response but sometimes a hyperpolarization or a biphasic membrane potential change was observed. Graded replacement of Cl by benzenesulphonate produced a progressive reduction in the depolarizing ability of noradrenaline and phenylephrine. When NaCl was replaced with Na isethionate or glucuronate, noradrenaline evoked small depolarizations in most of the cells but the sensitivity to noradrenaline (mV/nC) was reduced by over 80%. In other cells either no potential change or hyperpolarization was observed. In solutions containing low Cl the resting membrane potential was slightly depolarized by a mean value of 5-10 mV (depending on the anion substitute) and the membrane resistance was unaltered. In normal Krebs solution noradrenaline and phenylephrine evoked biphasic contractile responses but when NaCl was replaced by Na benzenesulphonate the initial rapid contraction was diminished greatly or abolished. However, the maximum tension produced by any concentration of noradrenaline was unaltered in low Cl solution although the contractions produced by phenylephrine were depressed in low Cl media. These data are further evidence that stimulation of the alpha 1-adrenoceptor in smooth muscle can produce contraction which is not mediated by a change in membrane potential.