cyclic-gmp and inositol-1-phosphate

Carbachol (50 microM) produced a rapid, transient increase in inositol-1,4,5-trisphosphate (IP3) levels in the rat anococcygeus; the peak increase observed at 10 s (3-fold above controls) was greatly reduced in the presence of atropine (100 nM), but was unaffected by nitrergic stimulation (10 Hz), sodium nitroprusside (10 microM) or 8-Br-cyclic GMP (200 microM). Following loading of muscles with [3H]myo-inositol, subsequent exposure to carbachol for 30 min resulted in a 6-fold increase in the accumulation of [3H]inositol-1-monophosphate; again, this action of carbachol was greatly attenuated by atropine, but unaffected by nitrergic stimulation, sodium nitroprusside or 8-Br-cyclic GMP. It is concluded that inhibition of agonist-induced generation of inositol phosphates cannot explain the ability of nitrergic activation to relax (by 54-62%) carbachol-induced tone in this tissue.

Topics: Anal Canal; Animals; Atropine; Carbachol; Cyclic GMP; Electric Stimulation; In Vitro Techniques; Inositol; Inositol 1,4,5-Trisphosphate; Inositol Phosphates; Male; Muscle, Smooth; Nitroprusside; Rats; Rats, Wistar

Clonidine and related imidazoline compounds bind to alpha 2-adrenergic as well as to newly described non-adrenergic imidazole/imidazoline receptors in brain and peripheral tissues. The present study was undertaken to identify the signal transduction mechanism coupled to this new class of receptors (imidazole receptors) using bovine adrenal chromaffin cells. Clonidine did not modify the basal or forskolin-stimulated production of cyclic AMP (cAMP), suggesting the absence of functionally active alpha 2-adrenergic receptors in adrenal chromaffin cells. Clonidine also failed to modify the basal and GTP gamma S- or carbachol-stimulated increase in phosphoinositide hydrolysis. However, clonidine increased significantly the production of cyclic GMP (cGMP) as well as the uptake of 45Ca2+. The cGMP response to clonidine was slower (peak at 15 min) and smaller (only about 50% over control) than the response to acetylcholine and was not shared by other agents that bind to imidazole receptors. In contrast, all agents that bind to imidazole receptors increased the influx of 45Ca2+ into chromaffin cells. It is concluded that (a) alpha 2-adrenergic and imidazole receptors are functionally distinct and linked to different signal transduction mechanisms; (b) the classical G-protein coupled soluble second messenger systems are not coupled to imidazole receptors; (c) clonidine may increase cGMP by a non-receptor-mediated intracellular action; and (d) imidazole receptors may regulate intracellular calcium levels through an ion regulating system that may be different from calcium channels.

Topics: Adrenal Glands; Animals; Calcium; Cattle; Cells, Cultured; Clonidine; Colforsin; Cyclic AMP; Cyclic GMP; Inositol Phosphates; Receptors, Drug; Second Messenger Systems; Signal Transduction

The metabolism of inositol phospholipids in response to epinephrine was investigated in intact human platelets. In platelets prelabelled with [3H]-myo-inositol in Ca2+-free HEPES buffer containing 10 mM LiCl, epinephrine caused an accumulation of inositol-1-phosphate in a concentration-dependent manner. The EC50 value for epinephrine was 5 microM. Yohimbine (1 microM), a selective alpha-2 adrenergic receptor antagonist, inhibited 88% of the epinephrine (10 microM) response, whereas prazosin (1 microM), a selective alpha-1 adrenergic receptor antagonist, failed to inhibit the response. Yohimbine inhibited the epinephrine (10 microM) response in a concentration-dependent manner. The inhibition constant (Ki) value for yohimbine was 60.3 nM. These data indicate that epinephrine stimulates phosphoinositide (PI) turnover by activating adrenergic receptors of the alpha-2 type in human platelets. In addition, this PI response elicited by epinephrine was found to be inhibited in a concentration-dependent manner by treatment of platelets with dibutyryl cyclic AMP and 8-bromo-cyclic GMP which are known as potent inhibitors for platelet activation, and may therefore be a useful biochemical index for the study of the function of human alpha-2 adrenergic receptors.

Topics: Blood Platelets; Bucladesine; Cyclic GMP; Epinephrine; Humans; Inositol; Inositol Phosphates; Mianserin; Prazosin; Receptors, Adrenergic, alpha; Sugar Phosphates; Yohimbine