To examine the effect of partial urethral obstruction (PUO) on bladder smooth muscle outward potassium current and the contribution of the large-conductance calcium-activated potassium (Maxi-K, BKCa) channel to this activity in smooth muscle cells isolated from bladders of sham-operated and PUO male rats using whole-cell patch clamp recording techniques. To determine the effect of PUO on the expression of the Maxi-K channel α and β1 subunits and in vitro detrusor contractility. Twenty adult male Sprague-Dawley rats were divided equally into two groups and subjected to surgical ligation of the urethra (PUO) or sham surgery (SHAM). After 2 weeks, the detrusors from PUO and SHAM rats were used for molecular analyses (mRNA and protein quantification of Maxi-K subunits) or organ bath contractility studies, or myocytes were isolated for conventional whole-cell patch clamp analyses. PUO increased bladder mass 2.5-fold and detrusor strips exhibited a more tonic-type contraction and increased contractility compared with controls (SHAM). Iberiotoxin (300 nM) sensitive Maxi-K channel current comprised about 40% of the outward whole-cell current in SHAM bladders but only about 8% in PUO bladders. Expression of the α subunit of the Maxi-K channel was significantly decreased ~40% while the expression of the β1 subunit was increased ~2-fold at the mRNA level. The increase in β1 expression was confirmed by Western blotting. Our findings show that obstruction of the rat bladder is associated with decreased Maxi-K channel activity of bladder smooth muscle cells, determined via direct current measurement. Increased expression of the β1 subunit points to a compensatory reaction to decreased Maxi-K channel activity. Maxi-K channel openers or gene therapy may therefore provide therapeutic benefit for the overactive bladder.
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