No abstract

SUMMARY1. The regulation of Ca2+-activated K+ channels by the agonist substance P in freshly dissociated smooth muscle cells from the rabbit longitudinal colonic muscle was characterized using the patch clamp technique.2. In the cell-attached recording mode, when pipette and bath solutions contained equal [K+] (126 mM), the Ca2+-activated K+ channels showed a linear current-voltage relationship (between -50 mV and 50 mV) with a slope conductance of 210 + 35 pS (n = 12). Reversal potential measurements indicated that the channel was highly selective for K+ (200,tM), nifedipine (10-6-10-5 M) or verapamil (10-6 M). The current was greatly reduced when the EGTA concentration in the pipette solution was increased from 0 77 to 10 mM.8. When the pipette solution contained CsCl, membrane depolarization activated inward currents. The peak inward current was identified as current through L-type Ca2+ channels based on its voltage-and time-dependent kinetics, and its modulation by dihydropyridines.9. The NK-1 receptor agonist substance P methyl ester (SPME) increased dose dependently (10-12-10-10 M) the amplitude and the rate of rise of the depolarizationactivated outward current at membrane potentials between -20 mV and 50 mV. This increase was abolished by lowering bath [Ca2+], and by adding Cd2+ (200 gM) or nifedipine (10-6 M) to the bath.10. When the pipette was filled with 140 mM-CsCl, substance P methyl ester (10-12-10-10 M) increased dose dependently the amplitude and the rate of decline of the voltage-activated Ca2+ current. No inward current was seen in the presence of nifedipine (106 M).11. These results indicate that substance P in the concentration range 10-12-10-10 M increases the open probability of Ca2+-activated K+ channels. This effect is predominantly mediated by the activation of L-type Ca2+ channels via interaction with a high-affinity neurokinin-1 (NK-1) receptor.

The membrane potential and contractile activity of colonic circular smooth muscle from New Zealand White rabbits were studied after the production of acute experimental colitis. Colitis was induced in the distal colon by rectal infusion of formaldehyde solution, followed by an intravenous bolus of soluble immune complexes. Despite active mucosal inflammation, there are only occasional inflammatory cells in the muscularis. Electrophysiological studies on tissue from control rabbits and rabbits with colitis were performed using double sucrose gap and intracellular microelectrode techniques. The resting membrane potential was lower (-44 +/- 3 mV) in muscle from rabbits with colitis compared with control animals (-54 +/- 2 mV) (P less than 0.02). Amplitude of the electrotonic potential after a hyperpolarizing current pulse was decreased (P less than 0.05) and the time constant was shortened (P less than 0.01) in muscle from animals with colitis compared with normal animals. Amplitude (13.1 +/- 2.3 mV) and maximum rate of rise (0.24 +/- 0.06 V/s) of the spike potential, initiated by a depolarizing current pulse, were decreased in muscle from animals with colitis compared with muscle from healthy animals (P less than 0.001). Isometric tension generation after electrical and chemical depolarization of the membrane or bethanechol administration was decreased (P less than 0.001) in muscle from colitic animals. These studies suggest 1) membrane resistance and membrane potential are decreased in muscle strips from animals with colitis; and 2) there is a disturbance in the electrical and mechanical response of these tissues after stimulation.

Myoelectric activity and intraluminal pressures were recorded simultaneously from the ileum, ileocecal sphincter (ICS), and proximal colon in chloralose-anesthetized cats. Slow-wave activity, seen at all areas, showed coupling of frequency in the distal ileum and ICS. ICS spike activity was both isolated and associated with ileal or colonic spike activity and correlated with phasic contractions (r = 0.86; P less than 0.01). Ileal distensions caused ICS relaxation and decreased spike activity 33.8% of the time. Colonic distensions caused contraction and increased spike activity 46.9% of the time. Migrating action-potential complexes (MAPC) induced by castor oil, ricinoleic acid, or cholecystokinin propagated to the ICS and through to the colon significantly more frequently than ileal non-MAPC (P less than 0.05). Both spike potential-dependent and spike potential-independent mechanisms were involved in ICS contraction. Bethanechol increased spike activity and phasic and tonic contractions. Phenylephrine, despite loss of spike activity in all leads, caused tonic contraction of the ICS. Isoproterenol caused loss of spike activity and decreased ICS pressure. Thus, ICS myoelectric activity appears to be important in determining sphincter function during neurohumoral and mechanical stimulation, with ICS contractions occurring through both a phasic spike-related mechanism and a tonic mechanism without spike activity.

The purpose of this study was to determine the length-tension properties and the response to neurohumoral agents of the feline ileocecal sphincter (ICS) circular smooth muscle in vitro. The ICS muscle during stretch maintained a higher resting tension at lower degrees of stretch than did the adjacent ileum and colon. Sodium nitroprusside (10(-3) M) or calcium-free Krebs solution with EGTA (5.0 mM) reduced the resting tension of ICS muscle by 15.8 +/- 2.1 and 19.1 +/- 2.5%, respectively. The ICS response to KCl depolarization was lower (0.37 +/- 0.03 kg/cm2) than colonic muscle (0.55 +/- 0.06 kg/cm2) oe ileal muscle (0.51 +/- 0.04 kg/cm2) (P less than 0.05). However, in response to acetylcholine, ICS muscle tension was minimal (0.06 +/- 0.02 kg/cm2) was compared with the colon (0.49 +/- 0.05 kg/cm2) or ileum (0.53 +/- 0.05 kg/cm2) (P less than 0.01). The ICS muscle contracted in response to phenylephrine, whereas other muscles were inhibited. Secretin, gastrin, glucagon, and cholecystokinin failed to act on the ICS muscle while other muscles did respond. These studies suggest that ICS muscle differs from adjacent ileal and colonic circular muscle in that a) in response to stretch it develops a high resting tension that is relatively resistant to calcium withdrawal, and b) it also shows quantitative and qualitative differences in response to neurohumoral agents.