Amiloride (AM) is a well known potassium sparing diuretic. The effects of AM at the cellular level include blockade of Na+/H+ exchange in several tissues and inhibition of passive sodium flux in epithelial cells. In this study we have explored the interactions of amiloride with muscarinic receptors, using isolated rat tracheal rings and compared its effects to those of the muscarinic receptor subtype-selective antagonist pirenzepine (PZ). The results obtained demonstrate the ability of AM (100 microM to 1mM) to inhibit the ACh induced rat tracheal contractions. The inhibition resulted in the reduction of the Emax values of ACh in this preparation, and the apparent Ki for AM was of 478 microM. This effect was also observed in a sodium-free choline medium, indicating that it is independent from sodium transport mechanisms sensitive to AM. In contrast to AM, PZ displayed a surmountable type of antagonism with a pA2 value of 6.52. The results demonstrate a differential antagonism by AM and PZ of the muscarinic receptors present in the smooth muscle of the rat trachea.
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