Second messengers involved in the muscarinic control of the heart: the role of the phosphoinositide response.

Two heart muscarinic responses are compared, the reduction of cyclic AMP concentration and the hydrolysis of phosphoinositides. It is suggested that the former is more important physiologically, since the latter is a slow response requiring much higher agonist concentrations. Inositol trisphosphate released from phosphoinositides is unlikely to produce a positive inotropic effect by releasing calcium from the sarcoplasmic reticulum. However, long term muscarinic effects may involve activation of protein kinase C by diacylglycerol released from phosphoinositides.

The muscarinic receptor of rat pituitary GH3 cells is coupled with adenylate cyclase inhibition, but not with phosphoinositide turnover.

The effects of muscarinic stimulation on cyclic AMP accumulation and on basal and thyrotropin-releasing hormone (TRH)-induced phosphoinositide turnover have been studied in rat pituitary GH3 tumour cells. Carbachol produced a dose-dependent atropine-sensitive inhibition of basal cyclic AMP accumulation. It had no effect on basal or TRH-stimulated production of inositol phosphates. The majority of the muscarinic receptors had a low affinity (Ki 0.22 microM) for pirenzepine and can be classified as M2 type, inhibiting adenylate cyclase but having no links with phosphoinositide metabolism.

Synthesis of some [N-(2-haloalkyl)amino]tetralin derivatives as potential irreversible labels for bovine anterior pituitary D2 dopamine receptors.

A series of hydroxylated 2-aminotetralins were prepared in the search for irreversible labels for D2 dopamine receptors. N-2-Haloacetyl and N-2-haloalkyl substituents were chosen as potential receptor alkylating groups. Titrimetric studies were carried out on [N-(chloroethyl)-N-methylamino]tetralins 10, 10a, 24, and 26 to demonstrate that aziridinium ions were formed as reactive intermediates from these compounds. This observation was confirmed by 1H NMR studies on compound 10. The majority of the aminotetralins prepared showed reasonably high affinity binding to anterior pituitary D2 dopamine receptors and exhibited agonist properties. Structure-activity results are presented together with preliminary studies designed to identify irreversible receptor binding agents. [N-(2-Chloroethyl)-N-propylamino]-6,7-dihydroxytetralin hydrobromide (18) proved most promising in these studies.

Guanine nucleotide effects on agonist binding at D2 dopamine receptors in bovine anterior pituitary.

[3H]Spiperone binding to D2 dopamine receptors on bovine anterior pituitary membranes is of high affinity, saturable and unaffected by guanine nucleotides. Antagonist displacement of [3H]spiperone binding is characterised by pseudo Hill coefficients close to one and is unaffected by guanine nucleotides. Agonist displacements show pseudo Hill coefficients less than one and agonist affinities are reduced by guanine nucleotides, although pseudo Hill coefficients are essentially unchanged. Agonist displacement curves may be treated in terms of two agonist binding sites with different agonist affinities, and in the presence of guanine nucleotides the two-binding site model remains the best description of the data.

Inhibition of inositol phospholipid breakdown by D2 dopamine receptors in dissociated bovine anterior pituitary cells.

Inositol phospholipids have been labelled with [3H]inositol in a lactotroph-enriched preparation of dissociated bovine anterior pituitary cells. Stimulation of cells with thyrotropin-releasing hormone receptor agonists leads to accumulation of [3H]inositol phosphates, and this effect may be inhibited by dopamine (DA) agonists. The DA agonist effect may be prevented by D2 DA receptor selective antagonists. Thus the D2 receptors on these cells are linked to inhibition of inositol phospholipid metabolism, and this provides a functional assay for the receptor.