In order to investigate the K+ channels and their effects on resting membrane potential (Em) and excitability in rat bronchial smooth muscle cells (BSMCs), the components of outward K+ channel currents and the effects of K+ channels on Em and tension in rat bronchial smooth muscle were observed by using standard whole-cell recording of patch clamp and isometric tension recording techniques. The results showed that under resting conditions, total outward K+ channel currents in freshly isolated BSMCs were unaffected by ATP-sensitive K+ channel blocker. There were two types of K+ currents: voltage-dependent delayed rectifier K+ channel (Kv) and large conductance calcium-activated K+ channel (BKc.) currents. 1 mmol/L 4-aminopyridine (4-AP, an inhibitor of Kv) caused a significant depolarization (from -8. 7±5. 9 mV to -25. 4±3. 1 mV, n=18, P<0. 001). In contrast, 1 mmol/L tetraethylammonium (TEA, an inhibitor of BKc.) had no significant effect on Em (from -37. 6±4. 8 mV to -36. 8±4.1mV, n=12, P>0. 05). 4-AP caused a concentration-dependent contraction in resting bronchial strips. TEA had no effect on resting tension, but application of 5 mmol/L TEA resulted in a left shift with bigger pD2(the negative logarithm of the drug concentration causing 50% of maximal effect) (from 6. 27±0. 38 to 6. 89±0. 54, n= 10, P<0. 05) in the concentration-effect curve of endothine-1, and a right shift with smaller pD2(from 8. 10±0. 23 to 7. 69±0. 08, n=10, P<0. 05) in the concentration-effect curve of isoprenaline. It was suggested that in rat BSMCs there may be two types of K+ channels, Kv and BKca, which serve distinct roles. Kv participates in the control of resting Em and tension. BKca is involved in the regulation of relaxation or contraction associated with excitation.
The effects of protein kinase C （PKC） on the tension and the activity of voltage-dependent delayed rectifier potassium channel （K,,） were examined in normal and passively sensitized human airway smooth muscle （HASM）, by measuring tones and whole-cell patch clamp techniques, and the Kv activities and membrane potential （Em） were also detected. The results showed that phorbol 12-myristate 13-acetate （PMA）, a PKC activator, caused a concentration-dependent constriction in normal HASM rings. The constriction of the passively sensitized muscle in asthma serum group was significantly higher than that of the normal group （P〈0.05）, and the constrictions of both groups were completely abolished by PKC inhibitor Ro31-8220 and calcium channel inhibitor nifedipine. Kv activities of HASM cells were significantly inhibited by PMA, and the Em became more positive, as compared with the DMSO （a PMA menstruum）-treated group （P〈0.01）. This effect could be blocked by Ro31-8220 （P〈0.01 ）. It was concluded that activation of PKC could increase the tones of HASM, which might be related to the reduced Kv activity. In passively sensitized HASM rings, this effect was more notable.
