Lack of agonistic activity in McN-A343 may be circumstantial

Differences in electrophysiological and contractile properties of mammalian cardiac tissues bathed in bicarbonate - and HEPES-buffered solutions

AIM

The aim of this study was to compare the action potential configuration, contractility, intracellular Ca2+ and H+ concentrations in mammalian cardiac tissues bathed with Krebs and Tyrode solutions at 37 degrees C.

RESULTS

In Langendorff-perfused guinea-pig hearts, loaded with the fluorescent Ca2+-indicator Fura-2, or H+-sensitive dye carboxy-SNARF, shifts from Krebs to Tyrode solution caused intra-cellular acidification, increased diastolic pressure and [Ca2+]i, decreased systolic pressure and [Ca2+]i, leading to a reduction in the amplitude of [Ca2+]i transients and pulse pressure. Contractility was also depressed in canine ventricular trabeculae when transferred from Krebs to Tyrode solution. Shifts from Krebs to Tyrode solution increased the duration of action potentials in multicellular cardiac preparations excised from canine and rabbit hearts but not in isolated cardiomyocytes. All these changes in action potential morphology, contractility, [Ca2+]i and [H+]i were readily reversible by addition of 26 mmol L(-1) bicarbonate to Tyrode solution. Effects of dofetilide and CsCl, both blockers of the delayed rectifier K current, on action potential duration were compared in Krebs and Tyrode solutions. Dofetilide lengthened rabbit ventricular action potentials in a significantly greater extent in Tyrode than in Krebs solution. Exposure of canine Purkinje fibres to CsCl evoked early after depolarizations within 40 min in all preparations incubated with Tyrode solution, but not in those bathed with Krebs solution.

CONCLUSION

It is concluded that the marked differences in action potential morphology, [Ca2+]i, [H+]i and contractility observed between preparations bathed with Krebs and Tyrode solutions are more likely attributable to differences in the intracellular buffering capacities of the two media.

Inhibition of field stimulation-induced contractions of rabbit vas deferens by muscarinic receptor agonists: selectivity of McN-A-343 for M1 receptors

Inhibition of the field stimulation-induced twitch responses of the rabbit vas deferens by the muscarinic receptor agonist, McN-A-343, has been attributed to presynaptic muscarinic receptors of the M1 subtype located on noradrenergic nerve terminals. Stimulation of these receptors causes inhibition of transmitter release and inhibition of the contractile response. However, the selectivity of McN-A-343 for M1 receptors has been questioned and this throws doubt on whether the prejunctional receptors of the rabbit vas deferens are of the M1 subtype. In this study we have undertaken a comprehensive re-evaluation of the inhibition of prostatic and epididymal portions of the rabbit isolated field-stimulated vas deferens by several agonists, including McN-A-343, and quantified the antagonism by M1-selective antagonists, pirenzepine and telenzepine. Prostatic and epididymal portions of vasa deferentia from New Zealand White rabbits were immersed in a low Ca2+ Krebs solution at 32+/-0.5 degrees C gassed with 5% CO2 in oxygen. Yohimbine (1.0mM) was present throughout to block prejunctional alpha2-adrenoceptors. Field stimulation was applied by repeated application of single pulses (30 V, 0.05 Hz, 0.5 ms) and isometric contractions recorded. Carbachol and oxotremorine initially potentiated the epididymal contractions but at higher concentrations there was inhibition. In the prostatic portion, oxotremorine only inhibited. McN-A-343 produced inhibitory responses only in both epididymal and prostatic portions. Pirenzepine shifted the concentration-response curves forthe inhibitory responses to oxotremorine to the right. However, the potentiation of the twitches also became more apparent with the lower concentrations of oxotremorine. Schild plots for the antagonism by pirenzepine yielded pA2 values of 7.96+/-0.004 and 7.7+/-0.02 for the epididymal and prostatic portions, respectively. The concentration-response curves for the inhibition of twitches by McN-A-343 were displaced to the right in a parallel manner by pirenzepine in both prostatic and epididymal portions with no potentiation of the twitches. The Schild plot for this antagonism generated pA2 values of 7.68+/-0.01 and 8.07+/-0.01, respectively. Telenzepine caused parallel shifts of the McN-A-343 concentration-response curves to the right in prostatic portions, the pA2 value being 8.70+/-0.13. Telenzepine (10(-7) M) abolished the inhibitory effect of carbachol to reveal only concentration-dependent potentiation of the contractions. The Schild plot for antagonism of this contractile effect yielded a pA2 value (7.07+/-0.09) that was significantly less by almost two orders of magnitude (1.70) than the value for the antagonism by telenzepine of the McN-A-343-induced inhibitory response. The pA2 values of pirenzepine and telenzepine against the inhibitory responses of the rabbit vas deferens are consistent with the involvement of M1 receptors. This leads to the conclusion that McN-A-343 causes inhibition through this receptor type. The doubts concerning the selectivity of McN-A-343 for M1 receptors are therefore unfounded. The fact that McN-A-343 does not display a selective binding profile suggests that its selectivity does not arise from affinity differences but probably resides in its intrinsic efficacy.

The interaction of McN-A-343 with muscarine receptors in cardiac and smooth muscle

The interaction of the muscarine receptor partial agonist (4-m-chlorophenylcarbamoyloxy)-2-butynyltrimethylammonium chloride (McN-A-343) was investigated at muscarine receptors in the atria and taenia caeci of the guinea-pig to compare its interaction at the muscarine M2 receptor in the two tissues. In the smooth muscle, the muscarine M3 receptor subtype is responsible for the contractile response but the major subtype detected in binding or antibody experiments is the M2 subtype. In guinea pig atria the dissociation constant of McN-A-343 at muscarine receptors was 15.2 microM determined in functional experiments on left atria in McEwen's solution or 14.8 microM in binding experiments with [3H]-(-)-quinuclidinyl benzilate ([3H]QNB) in the same medium containing 5'-guanylylimododiphosphate (50 microM). In the taenia caeci, the dissociation constant estimated for McN-A-343 at the M3 receptor from functional experiments based on the contractile response to the agonist in McEwen's solution was 4.6 microM. This value was similar to the dissociation constant (6.2 microM) estimated from binding studies versus [3H]QNB conducted in the same medium although studies with 11-[[2-[(diethylamino)methyl]-1-piperidinyl]acetyl]-5,11-dihydro-6H- pyrido[2,3-b][1,4]benzodiazepine 6-one (AF-DX 116) versus [3H]-(-)-N-methylscopolamine suggested that 70% of the receptors were the M2 subtype. The presence of the M2 subtype in the taenia caeci was also confirmed by the ability of oxotremorine to inhibit the increase in cAMP produced by isoprenaline (10 microM) since apparent pKB values for AF-DX 116 and hexahydrosiladiphenidol were 6.95 and 6.75, respectively. McN-A-343 (100 microM) failed to inhibit the response to isoprenaline and did not antagonize the inhibitory response to oxotremorine. It is concluded that the apparent affinity of McN-A-343 for muscarine M2 receptors in the atria and the taenia caeci differs and a number of explanations are discussed.