Comparative study of the molecular and functional expression of L-type Ca2+ channels and large-conductance, Ca2+-activated K+ channels in rabbit aorta and vas deferens smooth muscle

The relationship between the density of ionic currents through major two channels, voltage-dependent L-type Ca2+ channels (L-type VDCC) and large-conductance, Ca2+-activated Ki channels (BKC), and the mRNA expression levels of alpha 1C subunit of L-type VDCC (alpha 1C) and alp subunits of BKC (alpha BK/beta BK) were compared in smooth muscle cells (SMC) of rabbit aorta and vas deferens using whole cell-voltage clamp and reverse transcription-polymerase chain reaction (RT-PCR) analyses. The density of the currents through VDCC (I-Ca) and BKC (I-K,(Ca)) at +10 mV in aortic SMC was approximately one-seventh and one-sixth respectively of that in vas deferens. Whilst application of the Ca2+ channel agonist Bay K 8644 increased I-Ca by 75-90% in these SMC, the increase in I-K,I-Ca was far greater in aorta than in vas deferens. The expression of the alpha 1C transcript in vas deferens was approximately 3.5 times higher than that in aorta. In contrast, expression of alpha BK/beta BK was almost identical in both tissues, indicating the dissociation of I-K,I-Ca density from the expression levels of BKC transcripts in aorta. The results were supported by Western blot and immunocytochemical analyses using subunit-specific antibodies. The lower Ca2+ influx through VDCC in aorta activates only a very limited fraction of BKC compared with that in vas deferens. The greater expression of BKC than of VDCC in aortic SMC contributes to a strong negative feed-back mechanism that minimizes membrane depolarization and acts as a safety margin to maintain low membrane excitability.