Acetylcholine Induces Hyperpolarization Mediated by Activation of K (Ca) Channels in Cultured Chick Myoblasts

Our previous report demonstrated that chick myoblasts are equipped with Ca2+-permeable stretch- activated channels and Ca2+-activated potassium channels (KCa), and that hyperpolarization-induced by KCa channels provides driving force for Ca2+ influx through the stretch-activated channels into the cells. Here, we showed that acetylcholine (ACh) also hyperpolarized the membrane of cultured chick myoblasts, suggesting that nicotinic acetylcholine receptor (nAChR) may be another pathway for Ca2+ influx. Under cell-attatched patch configuration, ACh increased the open probability of KCa channels from 0.007 to 0.055 only when extracellular Ca2+ was present. Nicotine, a nAChR agonist, increased the open probability of KCa channels from 0.008 to 0.023, whereas muscarine failed to do so. Since the activity of KCa channel is sensitive to intracellular Ca2+ level, nAChR seems to be capable of inducing Ca2+ influx. Using the Ca2+ imaging analysis, we were able to provide direct evidence that ACh induced Ca2+ influx from extracellular solution, which was dramatically increased by valinomycin-mediated hyperpolarization. In addition, ACh hyperpolarized the membrane potential from -12.5+/-3 to -31.2+/-5 mV by generating the outward current through KCa channels. These results suggest that activation of nAChR increases Ca2+ influx, which activates KCa channels, thereby hyperpolarizing the membrane potential in chick myoblasts.

Physiological Roles and Properties of Ion Channels in Corporal Smooth Muscle / 대한남성과학회지

Decreased penile vascular resistance induced by corporal smooth muscle relaxation is the most important step in penile erection. The heightened tone of the corporal smooth muscles is considered a major cause in impotence. Modulation of corporal smooth muscle tone is a complex process requiring the integration of a host of intracellular events and extracellular signals. In intracellular events of corporal smooth muscle cell, the potassium channels and calcium channels play a major role. Functionally, potassium channels are important regulators of smooth muscle membrane potential in response to depolarizing stimuli and they counteract calcium channels. Potassium channels have been shown to play a fundamental role in both the physiologic and pathophysiologic regulation of smooth muscle tone in diverse tissues. Among the several subtypes of potassium channels, the calcium-sensitive potassium channel subtypes (KCa channel) and ATP-dependent potassium channel subtypes (KATP channel) are thought to be the most physiologically relevant in human corporal smooth muscle. With respect to intercellular communication, gap junction channels (connexin 43) are important in corporal smooth muscle cells. Thanks to gap junction channels, the signal of one cell can be spread to adjacent cells and coordination of corporal tissue response is possible. This report reviews the known details concerning junctional and nonjunctional ion channels in corporal smooth muscle and suggest the possibility of gene therapy targeting ion channels for erectile dysfunction

Role of K+ channels to resting membrane potential of rabbit middle cerebral arterial smooth muscle cells

The aim of the present study is to investigate the contribution of Ca2+-activated K+ (KCa) channels and delayed rectifier K+ (KV) channels to the resting membrane potential (RMP) in rabbit middle cerebral arterial smooth muscle cells. The RMP and membrane currents were recorded using the whole-cell patch configuration and single KCa channel was recorded using the outside-out patch configuration. Using the pipette solution containing 0.05 mM EGTA, the RMP was -25.76+/-5.08 mV (n=12) and showed spontaneous transient hyperpolarizations (STHPs). The membrane currents showed time- and voltage-dependent outward currents with spontaneous transient outward currents (STOCs). When we recorded the membrane potential using the pipette solution containing 10 mM EGTA, the RMP was depolarized and did not show STHPs. The membrane currents showed no STOCs but only showed slowly inactivating outward currents. External TEA (1 mM) reversibly inhibited the STHPs, depolarized the RMP, reduced the membrane currents, abolished STOCs, and decreased the open probability of single KCa channel. When KV currents were isolated, the application of 4-AP (5 mM) depolarized the RMP. The important aspect of our results is that KCa channel is responsible for the generation of the STHPs in the membrane potential and plays an important role in the regulation of the RMP and KV channel is also responsible for the regulation of the RMP in rabbit middle cerebral arterial smooth muscle cells.