Characterization of inhibitory innervation in porcine colonic circular muscle.

Effects of stimulation of intramural nerves in the circular smooth muscle layer of the porcine colon (Sus scrofa domestica) were studied using the sucrose-gap technique. Electrical field stimulation of the preparation, superfused with Krebs solution at 21 degrees C, induced a transient hyperpolarization of the smooth muscle cell membrane. This hyperpolarization was an inhibitory junction potential (IJP). The responses obtained from circular muscle originating from either the centripetal or centrifugal gyri of the ascending colon did not differ significantly. The IJP was characterized as being mediated by intramural, nonadrenergic, noncholinergic (NANC) nerves. The amplitude and latency of the IJP changed linearly with temperature (15-25 degrees C: +1 mV and -0.1 s per degree Celsius, respectively) reflecting a temperature-dependent synchronization of transmitter release. The membrane resistance decreased during the IJP. The IJP amplitude decreased or increased during conditioning hyperpolarizations or depolarizations, respectively, and reversed at membrane potentials about 30 mV more negative than the resting membrane potential. Potassium conductance blocking agents, barium (1 mM), tetraethylammonium chloride (TEA, 20 mM), 4-aminopyridine (4-AP, 5 mM), apamin (1 microM), and aminacrine (10(-4) M) added to the superfusion medium increased the membrane resistance. Only barium, TEA, and apamin depolarized the smooth muscle cell membrane. The IJP amplitude decreased in the presence of aminacrine and apamin to 75 and 35%, respectively, suggesting that apamin-sensitive Ca2+-activated K+ channels are involved in this response. ATP, adenosine, and related adenine nucleotides in concentrations up to 10(-3) M did not mimic the IJP. Superfusion with ATP for 15 min revealed a gradually increasing attenuation by up to 20% of the IJP. This might suggest that the release of neurotransmitter from intramural NANC nerves is modulated presynaptically via purinoceptors. Exogenously applied vasoactive intestinal polypeptide (VIP) in concentrations of 10(-9) to 10(-4) M did not affect the preparation. Also at elevated temperatures (up to 35 degrees C), VIP (10(-7) to 10(-4) M) did not cause measurable effects. It is concluded that the inhibitory mediator of the intramural NANC nerves present in the circular muscle layers of the porcine colon is neither a purine nor VIP.

Histamine H2-binding on guinea pig cerebral cortex.

The Kd-values of some histamine H2-active compounds, obtained from radio-ligand-binding studies on a homogenate of the guinea-pig cerebral cortex with 3H-tiotidine as the labelled H2-ligand, were compared with the pA2/pD2-value of these compounds on the guinea-pig right atrium and guinea-pig isolated gastric fundus. A good correlation was found between the pKd of the H2-antagonists and their pA2 on the guinea-pig right atrium. A much poorer correlation however was obtained between the pKd of the agonists on the cerebral cortex and their pD2-values on the guinea-pig right atrium and the gastric fundus. This poor correlation between true affinity and H2-activity of the agonists might be explained by spare receptors as a much better correlation was obtained between pKd and pD2 of partial agonists.

Differentiation of functional adrenoceptors in human and guinea pig airways.

The nature of the beta-adrenoceptor population(s) mediating direct smooth muscle relaxation, inhibition of antigen-induced histamine release and inhibition of antigen-induced (leukotriene-mediated) smooth muscle contraction of human and guinea pig central and peripheral airways was investigated. Preferential blockade by beta 1- and beta 2-selective antagonists of the relaxation induced by beta 1- and beta 2-selective agonists, respectively, revealed the guinea pig tracheal smooth muscle relaxation to be mediated by both beta 1- and beta 2-adrenoceptors. Using a highly beta 2-selective antagonist, the NE-induced relaxation was split up biphasically into a beta 1- and a beta 2-component. In contrast, no such differential blockade was observed with the relaxation of the guinea pig lung parenchyma strip, neither with the human tracheal, main bronchus and respiratory bronchiolus smooth muscle, which are all mediated by homogeneous beta 2-adrenoceptor populations. Only in the guinea pig trachea did neuronal and extraneuronal uptake inhibitors produce pronounced left shifts of the NE- and ISO-induced relaxation curves, respectively, suggesting a causal relationship between noradrenergic innervation and the presence of the beta 1-adrenoceptor subpopulation in the airways. Using the same techniques, it was established that inhibition of antigen-induced histamine release from guinea pig lung and tracheal mast cells is mediated by homogeneous beta 2-adrenoceptor populations as well. In contrast to catecholamines, non-catecholamine beta-agonists such as fenoterol, clenbuterol and zinterol had a substantially higher apparent affinity for the inhibition of the anaphylactic (leukotriene-mediated) guinea pig tracheal contraction than for the inhibition of histamine release; the same was true for lung tissue, though the difference was less pronounced. With some non-catecholamine beta-agonists considerable selectivity both in central and peripheral airway preparations was observed for the inhibition of anaphylactic contraction as compared with smooth muscle relaxation.

Comparison of functional beta-adrenoceptor heterogeneity in central and peripheral airway smooth muscle of guinea pig and man.

The nature of the beta-adrenoceptor population(s) mediating the relaxation of guinea pig and human airway smooth muscle was investigated. On the basis of a preferential blockade by beta 1- and beta 2-selective antagonists of the relaxation induced by beta 1- and beta 2-selective agonists, guinea pig tracheal strip relaxation was found to be mediated both by beta 1- and beta 2-adrenoceptors, the relative participation of which depending on the relative affinities of the agonist towards these two receptors. With highly selective antagonists the noradrenaline (NA)-induced relaxation could be split up biphasically into a beta 1- and a beta 2-component. In contrast, no such differential blockade was observed with the guinea pig lung parenchyma strip relaxation which is mediated by a homogenous beta 2-adrenoceptor population. On comparison of the tracheal, the spirally cut main bronchus- and intrapulmonary airway smooth muscle strips it could be shown that both the sensitivity of NA for neuronal uptake and the apparent affinity of the relaxation by NA decreased in the direction of the lung periphery. Using the same techniques it was ascertained that the relaxation of human tracheal smooth muscle (autopsy, obtained within 6 hours after death), main bronchus and intrapulmonary smooth muscle (operation) are mediated by homogenous beta 2-adrenoceptor populations. In addition, neuronal and extraneuronal uptake sites were not operative in these preparations, whether obtained from operation or from autopsy.