Toshifumi Kagiya

Cardiac M2 receptors consist of two different types, both regulated by GTP.

Cardiac muscarinic receptors are predominantly M2 receptors, and have three agonist binding sites (super-high(SH), high(H) and low(L) affinity agonist binding sites). Treatment of cardiac membranes with 50 nM propylbenzilyl choline mustard (PrBCM) caused 88% loss of binding sites for [3H]QNB. Carbamyl choline (CCh) inhibits this alkylation dose dependently and, theoretically, generates uneven alkylation of multiple agonist binding sites. Pretreatment of the membranes with 50 nM PrBCM and 0.5 mM CCh resulted in almost complete disappearance of L sites with similar degrees of conservation of H sites and SH sites. In these pretreated membranes, guanine nucleotide and sulfhydryl reagent caused a change in the ratio of residual SH and H sites but not of L sites though previous studies showed that, in intact membranes, these reagents affected the ratio of SH and L sites without significantly changing that of the H site. These results indicate the existence of two equilibria regulated by guanine nucleotide and sulfhydryl reagent in cardiac muscarinic receptors: one between SH and H sites and the other between H and L sites. The participation of GTP binding protein(s) in all cardiac muscarinic responses is suggested.

Multiple agonist binding sites of muscarinic acetylcholine receptors and their relation to the negative inotropic action of agonists in guinea-pig heart

The Kd values of the multiple agonist binding sites in cardiac muscarinic receptors (mAChR) and pD2 values for negative inotropic actions were determined independently and their relation was examined. The guinea-pig cardiac mAChR is known to have three agonist binding sites (super-high (SH), high (H) and low (L) affinity agonist binding sites) for carbachol (CCh). Pilocarpine (Pilo) and oxotremorine (Oxo) distinguished two sites (higher (Ho/p) with pKd of 5.88 and 8.20, respectively, and lower (Lo/p) affinity agonist binding sites with pKd of 5.08 and 6.17, respectively). The effects of guanine nucleotide and sulfhydryl reagent indicated that the Ho/p site corresponded with the SH site for carbachol, and the Lo/p site with the H + L sites for carbachol. The pD2 values of CCh, Pilo and Oxo for negative inotropic actions on autocontraction of right atria were 5.38, 5.30 and 6.80, respectively. The pD2 values of CCh and Oxo on electrically stimulated contraction of left atria in the presence of isoproterenol were 5.80 and 6.46, respectively, thus being closely related to H or Lo/p agonist binding sites of mAChR.

Detrimental effects of beta-adrenergic stimulation on beta-adrenoceptors and microtubules in the heart

SummaryIncreased plasma catecholamines — in particular, excessive beta-adrenoceptor activation in chronic heart failure — may easily desensitize the beta-adrenoceptors as well as the postreceptor signal transductions. Since these detrimental changes in the failing heart could be reversible, administration of low-dose beta-blocker, which minimizes the negative inotropic effects, may be effective in attenuating the harmful effects of sympathetic nerve activation. Beta-adrenoceptor stimulation may also produce microtubule disruptions of the cell either through direct action or through an increase in heart rate. Treatment with beta-blockers could attenuate Ca overload by slowing the heart rate and may be useful as a protection from the structural disintegration of the cell. Thus, to clarify the underlying mechanisms of betablocker therapy for chronic heart failure, we have to consider not only to the functional aspects but also to the structural changes of the cells.

Neurohumoral Mechanisms in Chronic Heart Failure

In chronic heart failure, sympathetic nerve activity is stimulated mainly due to blunted cardiopulmonary and arterial baroreceptors. Neural reflex from the working skeletal muscles during exercise accelerates the activation of sympathetic activity. The renin-angiotensin-aldosterone system is also activated in heart failure and induces vasoconstriction and retension of sodium ions and water. In the early stage of heart failure, these neurohumoral activations may play a compensatory role in cardiac dysfunction. In advanced heart failure, however, these mechanisms exert detrimental effects on myocardial cells and circulatory hemodynamics: (1) sympathetic stimulation and elevated angiotensin II and vasopressin induce vasoconstriction and increase the cardiac afterload, (2) retension of sodium ions and water by aldosterone may cause pulmonary congestion and peripheral edema, (3) sustained sympathetic stimulation produces down-regulation of β-adrenoceptors, and thus, inotropic responsiveness of the heart is attenuated, and (4) myocardial cells are directly injured by excessive catecholamines and angiotensin II. Recent clinical reports on the treatment of chronic congestive heart failure suggest that angiotensin converting enzyme inhibitors are effective for improvement of cardiac function, exercise capacity and prognosis. Long-term β-blocker therapy is also effective, although administration of β-blockers more or less depresses cardiac function in the initial phase. Through these clinical experiences, we should reappraise our interpretation of previous reports on the pathophysiology of chronic heart failure and strategy for treatment of chronic heart failure.

Effects of reserpine and adenosine agonist on α2-adrenergic receptor of rat vas deferens examined with [3H]yohimbine and [3H]clonidine

The changes of [(3)H]yohimbine and [(3)H]clonidine binding sites in rat vas deferens on treatments with adenosine receptor agonists (2-chloroadenosine, adenosine) or reserpine were examined. Treatment with adenosine agonist in vitro increased [(3)H]clonidine binding sites but had no influence on affinity and number of binding sites of ?(2)-antagonist, [(3)H]yohimbine. Amount of [(3)H]yohimbine binding sites was found to be higher than that of [(3)H]clonidine with or without the treatment. Inhibition curves of ?(2)-agonists, clonidine and norepinephrine, on [(3)H]yohimbine binding were less than unity though ?(2)-antagonist inhibited with about 1.0 of n(H). The treatment with adenosine agonist reduced the IC(50) value of agonists on the [(3)H]yohimbine binding but had no influence on the inhibitory effect of antagonist. These effect of adenosine agonists was completely blocked by theophylline. Accordingly it was considered that activation of adenosine receptor caused configurational change in ?(2)-adrenergic receptor from low affinity state for agonist to the high affinity state, though both states had same affinity for antagonist. On the other hand, treatment with reserpine in vivo increased the affinity of clonidine for ?(2)-adrenergic receptors and also increased the amount of the ?(2)-receptors.