Muscarinic cholinoceptors in the human heart: demonstration, subclassification, and distribution

Abstract

In human atrial and ventricular myocardium, the muscarinic cholinoceptor (M-cholinoceptor) populations were characterized by means of radioligand binding (with [N-methyl-3H]-scopolamine ([3H]-NMS) as the ligand) and functional experiments (negative inotropic effect of carbachol on isolated electrically driven right atrial and left papillary muscle preparations). (1) Binding of [3H]-NMS to human atrial and ventricular membranes was rapid, reversible and saturable (KD-values: 0.5-1.0 nmol/l). The maximal number of [3H]-NMS binding sites, however, was approximately 2.5-fold higher in right and left atrial membranes (200-250 fmol [3H]-NMS specifically bound/mg protein) than in right and left ventricular membranes (80-100 fmol/mg protein). (2) M-cholinoceptor antagonists inhibited [3H]-NMS binding to right atrial and left ventricular membranes with steep, monophasic competition curves indicating interaction with a single class of binding sites. In both tissues the order of potency was: atropine greater than AF-DX 116 greater than hexahydrosiladifenidol (HHSiD) greater than pirenzepine. (3) On isolated electrically driven right atrial and left papillary muscle preparations (with force of contraction enhanced by 10(-5) mol/l isoprenaline), carbachol (10(-8)-10(-4) mol/l) caused concentration-dependent decreases in force of contraction; the pD2-value for carbachol was 6.65 +/- 0.09 (n = 8, atria) and 6.62 +/- 0.08 (n = 10, papillary muscles). In both tissues M-cholinoceptor antagonists antagonized the negative inotropic effect of carbachol with an order of potency: atropine greater than AF-DX 116 greater than HHSiD greater than pirenzepine, identical to that obtained in radioligand binding experiments.(ABSTRACT TRUNCATED AT 250 WORDS)

Chronic β1‐adrenoceptor antagonist treatment sensitizes β2‐adrenoceptors, but desensitizes M2‐muscarinic receptors in the human right atrium

“…When a steady state of developed tension was reached -usually after 2-5 min -the next higher concentration was applied. When maximum contractile responses were obtained (usually at 3 x 10-7M procaterol, 3 x 10-6M isoprenaline, and 3 x 10-M noradrenaline), the 0-adrenoceptor agonists were added to reach final concentrations of 10-6M (procaterol), 10-'M (isoprenaline) or 10-4M (noradrenaline); under these conditions contractile force was stable for at least 40-60min (Deighton et al, 1990). Thereafter, cumulative concentration-effect curves for carbachol were carried out as described above.…”

“…The density of muscarinic receptors was determined by [N-Methyl-3H]-scopolamine ([3H]-NMS) binding at 7-9 concentrations of [3H]-NMS ranging from 0.1 to 10nM. Incubations were carried out for 60min at 25°C and terminated by adding 10ml of washing buffer to the entire incubation mixture, followed by rapid filtration over Whatman GF/C glass fibre filters that had previously been soaked in 1 mM unlabelled NMS (Deighton et al, 1990). Each filter was washed with an additional 10ml of washing buffer.…”

“…Cyclic AMP is formed by the enzyme adenylate cyclase, which is under dual control of stimulating and inhibitory receptors: for example, stimulation of cardiac /-adrenoceptors activates (Brodde et al, 1984;Gille et al, 1985;Kaumann & Lemoine, 1987;Bristow et al, 1989) and stimulation of muscarinic receptors inhibits adenylate cyclase (Waelbroeck et al, 1984). In the human heart, fl1-and 62-adrenoceptors coexist (for recent reviews see Brodde, 1987;Jones et al, 1989), whereas muscarinic receptors belong predominantly (if not exclusively) to the M2-subtype (Giraldo et al, 1988;Deighton et al, 1990). /i-Adrenoceptor antagonists are commonly used in the therapy of hypertension and angina pectoris (Scriabine, 1979;McDevitt, 1979).…”

Vagus nerve stimulation decreases left ventricular contractility in vivo in the human and pig heart

“…These studies show that in the human and pig heart the left vagus nerve can profoundly decrease the inotropic state of the left ventricular myocardium independent of its bradycardic effect. et al 1974;Loffelholz & Pappano, 1985;Du et al 1995) as well as a dense distribution of muscarinic M 2 receptors (Fields et al 1978;Goyal, 1989;Deighton et al 1990). However, a load-independent assessment of the influences of vagal nerve stimulation on contractility, in mammals other than the dog, is lacking.…”

PET receptors. Counting receptors using positron emission tomography.

“…Despite this limitation, these types of studies have led to important insights into receptor regulation in the heart. [1][2][3][4] Another approach has been to study circulating blood cells (leukocytes or platelets) as models for inaccessible organs. While easy to perform, these see p 1494 studies can be difficult to interpret, as receptors on blood cells can be regulated differently than those in the heart.5 Moreover, white blood cells are heterogeneous, and changes in receptor expression can be caused either by changes in intracellular receptor expression or a change in the population of cells that circulate in the blood.6 Furthermore, blood cells do not contain most of the receptors of interest in the heart; although platelets have a2-adrenergic receptors and leukocytes have /82-adrenergic and histamine receptors, muscarinic cholinergic, a1-adrenergic, ,Iladrenergic, and dopamine receptors do not appear to be present in large numbers on blood cells.…”