Ionic effects on amiloride block of the mechanosensitive channel in Xenopus oocytes

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Abstract
1 Patch clamp techniques were used to measure the ionic dependence of amiloride block of single mechanosensitive (MS) channels in frog (Xenopus laevis) oocytes. 2 The primary aim was to determine whether the difference in potency of amiloride block of MS channels in frog oocytes (IC50 = 0.5 mm) and chick auditory hair cells (IC50 = 50 μm) was due to the different ionic recording solutions. 3 Amiloride block of the oocyte MS channel does not vary significantly with complete substitution of external Na+ (i.e. 100 mm) with K+ in Ca2+-free recording solution (in both Na+ and K+ the IC50 = 0.5 mm). 4 A physiological concentration (1.8 mm) of external Ca2+ blocks the oocyte MS channel and reduces the potency of amiloride block (IC50 = 1.1 mm) without altering the voltage-dependence or the Hill coefficient (n = 1.8) of amiloride block. The reduction in potency can be explained by surface charge screening by Ca2+ which reduces the effective amiloride surface concentration. 5 The present results indicate that factors other than ionic recording conditions must underlie the difference in potency of amiloride block of MS channels in oocytes and auditory hair cells.