The stretch sensitivity of large conductance Ca²⁺-activated K⁺ channels (BK_Ca) in vascular smooth muscle has been considered a negative feedback in vasoconstriction. The activity of BK_Ca was shown to be modulated by the cytoskeleton, but whether the cytoskeleton regulates stretch sensitivity of BK_Ca is unknown. In the present study, the function of cytoskeletons in the regulation of BK_Ca activity and its stretch sensitivity was investigated.

Rabbits were anaesthetized with pentobarbital sodium (50 mg kg^-1). Using inside-out patch clamp technique, the single channel currents were recorded from enzymatically dispersed coronary arterial smooth muscle cells. Both the pCa 6.5 bath solutions and the pipette solution contained 150 mM KCl. BK_Ca was identified by the large unitary conductance (about 300 pS), by the voltage and calcium dependence, and by 100 nM iberiotoxin, a specific blocker of BK_Ca.

Disruption of actin filaments after brief treatment with cytochalasin D (1 µM), an actin filament disrupter, increased the open probability (NP_o) from 0.15 ± 0.03 to 0.26 ± 0.06 (mean ± S.E.M., n = 4, P < 0.01, Student’s paired t test). The increase in NP_o by cytochalasin D was largely reversed by phalloidin (1 µM), an actin filament stabilizer, to 0.03 ± 0.01 (n = 4, P < 0.01). A brief treatment with colchicine (10 µM), a microtubules disrupter, increased NP_o from 0.14 ± 0.03 to 0.31 ± 0.07 (n = 4, P < 0.01). A microtubules stabilizer, taxol (1 µM), markedly decreased NP_o to 0.04 ± 0.05 (n = 4, P < 0.01).

Applying -30 cmH₂O negative pressure to the pipette, NP_o of BK_Ca increased from 0.11 ± 0.03 to 0.54 ± 0.06 (n = 4, P < 0.01). With the stepwise increase (from -10 to -50 cmH₂O) of negative pressure, the activity of BK_Ca gradually increased without changing the unitary conductance.

In the presence of phalloidin, negative pressure hardly affected NP_o and NP_o at -30 and -40 cmH₂O was 0.07 ± 0.01 and 0.10 ± 0.01, respectively (n = 4). Taxol did not block the effect of negative pressure (at -30 cmH₂O, NP_o = 0.25 ± 0.04; at -40 cmH₂O, NP_o = 0.38 ± 0.05; n = 4).

So we could conclude that membrane stretch activates BK_Ca in coronary arterial smooth muscle cell. Actin filaments and microtubules modulate the activity of BK_Ca, but only actin filaments regulate the stretch sensitivity of BK_Ca.