ATP-sensitive and maxi potassium channels regulate BRL 37344-induced tocolysis in buffaloes-an in vitro study.

Cellular coupling of beta ₃ -adrenoceptors (β ₃ -ADR) to potassium channels in myometrium is largely unknown. In vitro study was undertaken to unravel the presence of β ₃ -adrenergic receptors (ADR) and the role of K ⁺ -channels in mediating β ₃ -ADR-induced relaxation in isolated myometrial strips from cyclic non-pregnant water buffaloes. Isometric tension was recorded in isolated myometrial strips using data acquisition system based physiograph. Compared to SR 59230A, BRL 37344 was found to be more potent in inducing β ₃ -dependent myometrial relaxation which was significantly (p < 0.05) inhibited in the presence of β ₃ antagonist, SAR 150640. The immunoreactive protein to β ₃ -ADR was also detected in membrane fraction of myometrial protein. Further, incubation with BRL 37344 (10 μM) significantly (p < 0.05) increased c-AMP accumulation (37.58 ± 9.52 pmol/mg protein; n = 4) in the myometrial strips compared to basal c-AMP level (16.85 ± 3.87 pmol/mg protein; n = 4). The concentration response curves (CRC) of BRL 37344 were significantly (p < 0.05) shifted towards right in the presence of K _ATP channels specific blocker, glibenclamide (10 μM) and maxi K ⁺ -channels (BK _Ca ) specific blocker, iberiotoxin (100 nM), with decrease in both efficacy and potency as compared to control. However, 4-aminopyridine (4-AP), a specific blocker of the voltage gated K ⁺ -channels (Kv), failed to alter the CRC of BRL 37344. Existence of immunoreactive protein to Kir6.1, α-subunit of BK _Ca and Kv1.1 channels were also detected in the membrane fraction of myometrial protein. Based on the above findings, it can be concluded that BRL 37344 is a potent stimulator of β ₃ -adrenoceptors in buffalo myometrium and besides mediating their effect through rise in c-AMP, they are coupled to K _ATP and BK _Ca channels in inducing tocolytic effects.
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