Many organs containing smooth muscle are myogenically active. This was assumed to originate from activity within the individual smooth muscle cells. Some smooth muscle cells have low resting membrane potentials and generate myogenic activity, in much the same way as cardiac pacemaker cells, through the sequential activation of voltage-dependent ion channels (see for example Anderson, 1993). In others, myogenic activity originates from the cyclic release of calcium ions (Ca¥) from stores inside the smooth muscle cells (Van Helden, 1993;Hashitani et al. 1996). Many regions of the gastrointestinal tract generate slow waves and contract rhythmically at low frequencies in the absence of stimulation (Tomita, 1981;Sanders, 1992). Again it was initially thought that the generation of slow waves reflected some properties of gastrointestinal smooth muscle cells (Connor et al. 1974;El-Sharkaway & Daniel, 1975;Tomita, 1981). More recently it has been suggested that slow waves result from the interaction between two distinct groups of cells: one group acts as pacemaking cells and activates a second group which generates slow waves. Several observations suggest that activity originates in interstitial cells of Cajal (ICC), and that smooth muscle cells, rather than initiating activity, act as follower cells. ICC form diffuse networks of cells which are thought to be linked together as electrical syncytia (Thuneberg, 1982). When ICC lying near the submucous border of the circular muscle layer of dog colon are dissected away, nearby smooth muscles stop generating slow waves (Smith et al. 1987). Intestinal preparations taken from mice devoid of ICC fail to generate normal slow waves (Ward et al. 1994;Huizinga et al. 1995).

SUMMARY1. The cable properties of smooth muscle of guinea-pig taenia coli were studied by intracellular recording of electrotonic potentials produced by square current pulses and alternating current applied with external electrodes.2. An electrical model of the smooth muscle was constructed to test how the junctional resistance between cells affected the cable properties. The model consisted of a series of short cables (representing cells) which were connected by junctional resistances.3. It was concluded, from the experiments on the living tissue and on the model, that the electrotonic potential in smooth muscle can be expressed by the ordinary cable equation used for nerve and skeletal muscle fibres, even though the junctional resistance is of the samne order of magnitude as that of the myoplasmic resistance.4. The cable equation was used to analyse the membrane parameters from the electrotonic potential, from the time course of the foot of the spike and from the conduction velocity. The analysis indicates that the smooth muscle has a membrane capacity of 2-3 ,uF/cm2 and a membrane resistance of 30-50 kQ cm2.

Isoprenaline is a beta-adrenergic agonist of clinical importance as a remedy for asthma. In airway smooth muscle its relaxant action is accompanied by hyperpolarization of the membrane and elevation of the level of intracellular cyclic AMP. Hyperpolarization and relaxation are also induced by drugs such as forskolin, theophylline and dibutyryl cAMP, indicating that cAMP-dependent phosphorylation is involved in producing the electrical response. Cyclic AMP-dependent protein kinase (protein kinase A) has been reported to activate Ca2+-dependent K+ channels in cultured aortic smooth muscle cells and snail neurons. The membrane of tracheal smooth-muscle cells is characterized by a dense distribution of Ca2+-dependent K+-channels. We have now examined the effect of isoprenaline and protein kinase A on Ca2+-dependent K+-channels in isolated smooth muscle cells of rabbit trachea, using the patch-clamp technique. Our results show that the open-state probability of Ca2+-dependent K+-channel of tracheal myocytes is reversibly increased by either extracellular application of isoprenaline or intracellar application of protein kinase A. We also show that this effect is significantly enhanced and prolonged in the presence of a potent protein phosphatase inhibitor, okadaic acid.

SUMMARY1. The electrical responses of single smooth muscle cells of the guineapig taenia coli to external stimulation were studied in two times hypertonic solution and compared with the responses to intracellular stimulation.2. Exposure to Krebs solution made two times hypertonic by adding sucrose abolished the mechanical movement and stopped the spontaneous electrical activity. The electrical response to stimulation was essentially similar to that in physiological solution.3. When the tissue was placed between stimulating electrodes, the cells near the cathode were depolarized and produced spikes, while the cells near the anode were hyperpolarized and produced small spikes only with weak stimuli. The cells near the centre were not polarized but produced spikes with a frequency pattern similar to that near the cathode.4. When both stimulating electrodes were put close together at one end of the tissue, the intracellularly recorded extrapolar polarization changed its polarity at 1-2 mm distance from the stimulating electrode. When an insulating partition was placed between the stimulating and recording site, the reversed polarity was no longer observed and the electrotonic potential spread decayed roughly exponentially with distance from the stimulating electrode. The time course of the electrotonic potential was similar to that predicted from the cable equation applied to nerve. The space constant was 1-68 + 0-08 mm (S.E. of mean) and the time constant was 60-100 msec. The cable properties may be explained by assuming that many fibres, connected in series and in parallel, are aggregated as functional units.5. The strength-duration curve was a simple hyperbola and the chronaxie was about 20 msec. The relation between extracellularly applied current and intracellularly recorded potential showed that membrane resistance decreased with depolarization and slightly increased with EXTERNAL STIMULATION OF SMOOTH MUSCLE 451 hyperpolarization. The spike was propagated in both directions at the same speed as in physiological solution (7.3 + 0 7 cm/sec).6. Long anodal current often produced electrical activity of low amplitude which seemed to be due to the spike activity near the cathode, because the same frequency modulation was seen in both activities, and external hyperpolarization reduced the size of the propagated spike.

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