Regulation of the Ca²⁺-sensitive domains of the maxi-K channel in the mouse myometrium during gestation

Large conductance Ca²⁺-activated K⁺ channels (maxi-K channels) are known to modulate uterine activity during gestation. Electrophysiological recordings demonstrate that myometrial maxi-K current is suppressed in term-pregnant compared to non-pregnant mice. We sought to determine whether maxi-K current suppression is due to reduction of maxi-K channel protein or differential expression of maxi-K channel isoforms that vary in their Ca²⁺ and voltage sensitivities. Immunoblot analyses show an increase of maxi-K channel protein throughout gestation. Polymerase chain reaction of mouse myometrial cDNA identified four alternatively spliced sites within the maxi-K transcript and three within the Ca²⁺-sensitive 'tail' domain. Ribonuclease protection analyses demonstrate that total channel transcript levels mimic protein expression; however transcript levels of alternatively spliced regions of regulatory domains that alter sensitivity to voltage and Ca²⁺ differ in their gestational expression. An insert that increases the maxi-K channel sensitivity to voltage and Ca²⁺ is present at steady levels throughout gestation, differing from total channel transcript regulation. The insert-less form of this transcript, which reduces the channel voltage and Ca²⁺ sensitivity, is not detected until midterm pregnancy. These findings verify that multiple isoforms of the maxi-K channel are present in the mouse myometrium and are regulated differentially during gestation, which is a likely mechanism for modulation of myometrial excitability during pregnancy.