The H-L subgroup of guinea-pig cardiac M2 receptors (M2 beta) regulates inositol phosphate formation

Calcineurin-NFATc regulates type 2 inositol 1,4,5-trisphosphate receptor (InsP3R2) expression during cardiac remodeling

In heart, the type 2 inositol 1,4,5-triphosphate receptor (InsP3R2) is the predominant isoform expressed and is localized in the nuclear membrane of ventricular myocytes. InsP3R2-mediated Ca(2+) release regulates hypertrophy specific gene expression by modulating CaMKIIδ, histone deacetylase, and calcineurin-NFATc signaling pathways. InsP3R2 protein is a hypertrophy specific marker and is overexpressed in heart failure animal models and in humans. However, the regulation of InsP3R2 mRNA and protein expression during cardiac hypertrophy and heart failure is not known. Here we show the transcriptional regulation of the Itpr2 gene in adult cardiomyocytes. Our data demonstrates that, InsP3R2 mRNA and protein expression is activated by hypertrophic agonists and attenuated by InsP3R inhibitors 2-aminoethoxyldiphenyl borate and xestospongin-C. The Itpr2 promoter is regulated by the calcineurin-NFATc signaling pathway. NFATc1 regulates Itpr2 gene expression by directly binding to the Itpr2 promoter. The calcineurin-NFATc mediated up-regulation of the Itpr2 promoter was attenuated by cyclosporine-A. InsP3R2 mRNA and protein expression was up-regulated in calcineurin-A transgenic mice and in human heart failure. Collectively, our data suggests that ITPR2 and hypertrophy specific gene expression is regulated, in part, by a positive feedback regulation between InsP3R2 and calcineurin-NFATc signaling pathways.

Molecular and functional identification of m1 muscarinic acetylcholine receptors in rat ventricular myocytes

The expression of muscarinic acetylcholine receptor (mAChR) subtypes in freshly isolated adult rat ventricular myocytes was investigated by reverse transcription of cellular mRNA followed by amplification of cDNA using the polymerase chain reaction (PCR). After reverse-transcriptase PCR, bands were obtained corresponding to the expected sizes for the m1 and m2 but not for the m3 to m5 mAChRs. The identity of the m1 and m2 bands was confirmed by single-cell PCR, restriction digest mapping, and Southern blot analysis. The presence of m1 and m2, but not m3, mAChR protein in these cells was shown by indirect immunofluorescence studies using subtype-specific antibodies. It was further investigated whether the identified m1 mAChR was responsible for the stimulatory effects on Ca2+ transients by high concentrations of carbachol ( > 10 mumol/L) known to occur in these cells. In pertussis toxin-treated ventricular myocytes electrically stimulated at 1 Hz, carbachol (300 mumol/L) increased the basal Ca2+ level from 96 +/- 7 to 118 +/- 8 nmol/L and the peak Ca2+ transient level from 519 +/- 32 to 640 +/- 36 nmol/L (mean +/- SEM P < .05 for both, n = 8). These effects of carbachol on Ca2+ transients were antagonized by 10 nmol/L pirenzepine, an m1 mAChR-selective antagonist. In contrast, the m2 mAChR-selective antagonist methoctramine (up to 100 nmol/L) did not inhibit the response. These results are the first to use single-cell PCR to probe cardiomyocyte-specific gene expression and indicate that m1 mAChRs are expressed on adult rat ventricular myocytes in addition to m2 mAChRs. The results further suggest that m1 mAChRs mediate the stimulatory responses on Ca2+ transients to high concentrations of cholinergic agonists seen in these cells.

Analysis of muscarinic cholinoceptors mediating phosphoinositide hydrolysis in guinea pig cardiac muscle

The muscarinic receptor mediating stimulation of PI hydrolysis in guinea pig atria and ventricles has been studied. The non-selective muscarinic agonist (+)-cis-dioxolane elicited this response, concentration-dependently, with a potency indicative of a low receptor reserve. The potency of a novel, M2-selective agonist, L-660,863 (-log EC50 = 6.3, atria; 6.0, ventricles) was observed to be lower than its apparent affinity (-log KA = 7.6) for M2 receptors, indicating an action probably mediated by a population distinct from that producing negative inotropy in the same tissue. The inhibition of the response to (+)-cis-dioxolane by several muscarinic antagonists (atropine, pirenzepine, AF-DX 116, methoctramine, HHSiD and pFHHSiD) generated an affinity profile for this receptor also dissimilar to that described for the receptor mediating the classical cardiac 'M2' response. Although no other muscarinic receptor mRNA has been detected in this tissue, these data suggest the presence of a second population of muscarinic sites, which may signify an M2 receptor diversity.

Characterization of muscarinic receptor subtypes in canine left ventricular membranes

The pharmacological characteristics of muscarinic receptor (mAChR) subtypes in canine left ventricular membranes (LVM) were determined using [3H] quinuclidinyl benzilate ([3H]QNB) and [3H]N-methyl scopolamine ([3H]NMS) as ligands. Binding of [3H]QNB and [3H]NMS was saturable with respect to the radioligand concentrations. Analysis of binding isotherms by Scatchard plot showed that [3H]QNB and [3H]NMS bound to an apparently homogeneous population of mAChRs in LVM, with KD values of 390 +/- 100 and 285 +/- 34 pM and Bmax values of 240 +/- 20 and 133 +/- 9 fmol/mg protein, (n = 6), respectively. The Hill coefficients for [3H]QNB and [3H]NMS binding were 0.95 +/- 0.02 and 0.99 +/- 0.01, respectively. Based on the competitive inhibition of [3H]ligand binding, atropine and NMS as well as the selective M1 antagonist PZ revealed no selectivity for these mAChRs. PZ competed with [3H]QNB or [3H]NMS for a single binding site with a Ki value of 0.23 +/- 0.03 microM and 0.62 +/- 0.10 microM, (n = 6), respectively, which is close to the values of M2 or M3 receptors. The data indicate that the M1 receptor subtype did not exist in canine LVM. Competition of [3H]ligand binding with selective M2 antagonists, AF-DX 116 and methoctramine and the selective M3 antagonists, 4-DAMP and hexahydrosiladifenidol, gave a best fit for a two-binding site model. The inhibition of carbachol-mediated phosphoinositide hydrolysis by PZ, AF-DX 116 and 4-DAMP, generated an affinity profile for this response also dissimilar to that described for the classical cardiac M2 response. Although no other muscarinic receptor mRNA has been detected in this tissue, these data suggest the presence of a second population of muscarinic sites, which may signify an M2 receptor diversity.