Abstract
The role of aging on contraction or relaxation through muscarinic or alpha-adrenergic receptors, respectively, was studied in isolated rat jejunum. Furthermore, the influence of extracellular calcium was analyzed, through functional and radioligand binding assays. The rank order of potency for selective muscarinic antagonists for M(1), M(2), and M(3) receptor subtypes, measured from affinity (pA(2)) values, was p-fluorohexahydrosiladifenidol (pFHHSiD) (M(3)) > pirenzepine (M(1)) > methoctramine (M(2)), indicating a predominance of M(3) subtype. This order was unchanged with age. Contractions by muscarinic agonist methacholine (MCh) were diminished in aged rats, resulting in lower apparent affinity (pD(2)) values, compared with adult controls. A larger decrease of MCh contractions occurred in aged rats after Ca(2+) withdrawal or after the calcium channel blocker isradipine. Changes were not detected for relaxation by adrenergic agonists. In conclusion, aging caused a decrease of MCh potency, which is probably related to the reduction of calcium sensitivity in jejunum.
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