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Keywords:

  • Muscarinic receptor subtypes;
  • Cerebral blood vessel (cat);
  • Vasodilatation;
  • Vasoconstriction;
  • Phosphoinositide turnover;
  • Endothelium;
  • Second messenger

Abstract: The muscarinic receptors involved in phosphoinositide (PI) hydrolysis have been pharmacologically characterized in cat cerebral blood vessels. Carbachol elicited a concentration-dependent increase in inositol phosphate accumulation [inositol monophosphate, bisphosphate, trisphosphate (IP3) and tetrakisphosphate] in both major cerebral arteries and small pial vessels, which reached 140–280% of baseline at 10−3M carbachol (referred to as maximal effect). However, the inositol phosphate accumulation response was found to be biphasic with a submaximal effect (30–50% of the maximal stimulation) obtained at low carbachol concentrations (<10−5M). Endothelial denudation induced a virtual disappearance of the submaximal PI response without affecting that elicited by high concentrations of carbachol. The pharmacology of the two carbachol-induced PI responses was investigated by comparing the potency of selected muscarinic antagonists to block the IP3 accumulation induced by 10−7M (endothelium-dependent submaximal effect) and 10−4M (endothelium-independent near-maximal effect) carbachol. In both major arteries and pial vessels, the activation of IP3 production by 10−4M carbachol was similarly inhibited by muscarinic antagonists with the following averaged rank order of potency (in -log IC50): 4-diphenylacetoxy-N-methylpiperidine methiodide (4-DAMP; 8.65) > pirenzepine (8.28) > 6-chloro-5,10-dihydro-5-[(1-methyl-4-piperidinyl)acetyl]-11H-dibenzo[b,e][1,4]diazepine-11-one (UH-AH 371; 7.87) > 11-[[2-[(diethylamino)methyl]-1-piperidinyl]acetyl]-5,-11-dihydro-6H-pyridol[2,3-b][1,4]benzodiazepine-6-one (AF-DX 116; 6.62), a pharmacological profile compatible with an M1 receptor subtype. In contrast, the submaximal stimulation of the PI metabolism elicited by 10−7M carbachol in major arteries was blocked by the same antagonists with the following order of potency (in -log IC50): 4-DAMP (8.38) > pirenzepine (7.25) > UH-AH 371 (6.25) > AF-DX 116 (5.72), which was reminiscent of an M3 pharmacological profile. These findings indicate that stimulation of cerebrovascular muscarinic receptors is accompanied by PI hydrolysis via two distinct receptors, most probably the M1 and M3 subtypes that have been associated with constriction and dilatation, respectively, of cat cerebral arteries. Furthermore, these results provide strong evidence for an endothelial localization of the M3 dilatatory receptors within the vessel wall.