LY-294002-inhibitable PI 3-kinase and regulation of baseline rates of Na+ transport in A6 epithelia

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Abstract
Blocker-induced noise analysis of epithelial Na+ channels (ENaCs) was used to investigate how inhibition of an LY-294002-sensitive phosphatidylinositol 3-kinase (PI 3-kinase) alters Na+transport in unstimulated and aldosterone-prestimulated A6 epithelia. From baseline Na+ transport rates ( I Na) of 4.0 ± 0.1 (unstimulated) and 9.1 ± 0.9 μA/cm2 (aldosterone), 10 μM LY-294002 caused, following a relatively small initial increase of transport, a completely reversible inhibition of transport within 90 min to 33 ± 6% and 38 ± 2% of respective baseline values. Initial increases of transport could be attributed to increases of channel open probability ( P o) within 5 min to 143 ± 17% (unstimulated) and 142 ± 10% of control (aldosterone) from baseline P o averaging near 0.5. Inhibition of transport was due to much slower decreases of functional channel densities ( N T) to 28 ± 4% (unstimulated) and 35 ± 3% (aldosterone) of control at 90 min. LY-294002 (50 μM) caused larger but completely reversible increases of P o (215 ± 38% of control at 5 min) and more rapid but only slightly larger decreases of N T. Basolateral exposure to LY-294002 induced no detectable effect on transport, P o or N T. We conclude that an LY-294002-sensitive PI 3-kinase plays an important role in regulation of transport by modulating N T and P o of ENaCs, but only when presented to apical surfaces of the cells.