Differential modulation of Ca2(+)-activated K+ channels by substance P

Ketamine blocks Ca2+-activated K+ channels in rabbit cerebral arterial smooth muscle cells

Although ketamine and Ca2+-activated K+ (KCa) channels have been implicated in the contractile activity regulation of cerebral arteries, no studies have addressed the specific interactions between ketamine and the KCa channels in cerebral arteries. The purpose of this study was to examine the direct effects of ketamine on KCa channel activities using the patch-clamp technique in single-cell preparations of rabbit middle cerebral arterial smooth muscle. We tested the hypothesis that ketamine modulates the KCa channel activity of the cerebral arterial smooth muscle cells of the rabbit. Vascular myocytes were isolated from rabbit middle cerebral arteries using enzymatic dissociation. Single KCa channel activities of smooth muscle cells from rabbit cerebral arteries were recorded using the patch-clamp technique. In the inside-out patches, ketamine in the micromolar range inhibited channel activity with a half-maximal inhibition of the ketamine concentration value of 83.8 +/- 12.9 microM. The Hill coefficient was 1.2 +/- 0.3. The slope conductance of the current-voltage relationship was 320.1 +/- 2.0 pS between 0 and +60 mV in the presence of ketamine and symmetrical 145 mM K+. Ketamine had little effect on either the voltage-dependency or open- and closed-time histograms of KCa channel. The present study clearly demonstrates that ketamine inhibits KCa channel activities in rabbit middle cerebral arterial smooth muscle cells. This inhibition of KCa channels may represent a mechanism for ketamine-induced cerebral vasoconstriction.

Spatial and temporal patterns of intracellular calcium in colonic smooth muscle

Intracellular calcium [Ca2+]i measurements in cell suspension of gastrointestinal myocytes have suggested a single [Ca2+]i transient followed by a steady-state increase as the characteristic [Ca2+]i response of these cells. In the present study, we used digital video imaging techniques in freshly dispersed myocytes from the rabbit colon, to characterize the spatiotemporal pattern of the [Ca2+]i signal in single cells. The distribution of [Ca2+]i in resting and stimulated cells was nonhomogeneous, with gradients of high [Ca2+]i present in the subplasmalemmal space and in one cell pole. [Ca2+]i gradients within these regions were not constant but showed temporal changes in the form of [Ca2+]i oscillations and spatial changes in the form of [Ca2+]i waves. [Ca2+]i oscillations in unstimulated cells (n = 60) were independent of extracellular [Ca2+] and had a mean frequency of 12.6 +/- 1.1 oscillations per min. The baseline [Ca2+]i was 171 +/- 13 nM and the mean oscillation amplitude was 194 +/- 12 nM. Generation of [Ca2+]i waves was also independent of influx of extracellular Ca2+. [Ca2+]i waves originated in one cell pole and were visualized as propagation mostly along the subplasmalemmal space or occasionally throughout the cytoplasm. The mean velocity was 23 +/- 3 microns per sec (n = 6). Increases of [Ca2+]i induced by different agonists were encoded into changes of baseline [Ca2+]i and the amplitude of oscillations, but not into their frequency. The observed spatiotemporal pattern of [Ca2+]i regulation may be the underlying mechanism for slow wave generation and propagation in this tissue. These findings are consistent with a [Ca2+]i regulation whereby cell regulators modulate the spatiotemporal pattern of intracellularly generated [Ca2+]i oscillations.

Ca-dependent K channels in smooth muscle cells permeabilized by beta-escin recorded using the cell-attached patch-clamp technique

Using the cell-attached patch-clamp technique, the activity of single, Ca-dependent K channels was recorded in single smooth muscle cells permeabilized by beta-escin. The conductance and the relationship between the open probability of the channels and pCa recorded in permeabilized cells were very similar to those obtained in excised inside-out patches. At pCa 7, application of 30 microM acetylcholine (ACh) or 0.1 microM substance P (SP) together with 1 mM guanosine 5'-trisphosphate to permeabilized cells elicited transient bursts of channel openings similar to those which occur in intact cells. Transient activation was also observed when 2-30 microM inositol trisphosphate (IP3) was applied to permeabilized cells. This single channel activity was inhibited by pretreatment with low-molecular-weight heparin at 50-100 micrograms/ml. Channel activity at pCa 7.0 was greatly enhanced by 200 microM cyclic adenosine monophosphate. These results provide direct evidence that single Ca-dependent K channel activity is regulated by the transmitters ACh and SP, as well as a second messenger, IP3, via the release of intracellular Ca from intracellular sites which are blocked by heparin. This novel approach is valuable in elucidating second messenger mechanisms involved in the regulation of single channel activity by transmitters and autocoids, since permeabilization by beta-escin preserves the entire system of receptor-operated signal transduction and allows intracellular application of second messengers at fixed concentrations.

Characterization of large-conductance chloride channels in rabbit colonic smooth muscle

1. A large-conductance Cl- channel was characterized in cell-free membrane patches from the rabbit longitudinal colonic smooth muscle using the patch clamp technique. In addition, the regulation of these channels by neurokinin-1 (NK-1) receptor agonists and G proteins was studied. 2. No spontaneous channel activity was observed in cell-attached patches at the cell resting potential, or in excised patches at pipette potentials (Vp) between -20 and 20 mV. In excised patches, channel activity could be induced in thirty-six out of ninety-six patches by holding the patch at Vp values more negative than -60 mV or more positive than 60 mV. Once induced, the channel showed a bell-shaped voltage activation curve in high symmetric [Cl-], with maximal open probability between 20 and -5 mV. Varying cytosolic calcium concentration ([Ca2+]) between 5 x 10(-8) M and 1.0 mM had no effect on the voltage activation of the channel. 3. In inside-out and outside-out patches, when pipette and bath solutions contained equal [Cl-] (130 mM), the anion channel showed a linear current-voltage (I-V) relationship between -60 and 60 mV with a slope conductance of 309 +/- 20 pS (n = 13). Reversal potential measurements indicated that the channel was selective for Cl- over Na+ and K+ (PCl/PNa = 6:1). 4. Channel openings from the closed state to the full open state as well as transitions through smaller conductance states were observed. The smallest detectable substate had a conductance of 15.6 pS. Based on the similarities in selectivity and linearity of the I-V curve of the smaller conductances with the full open state, and kinetic analysis of channel activity, it is concluded that the large conductance channel is composed of multiple substates which can either open and close independently, or simultaneously via a main gate. 5. The stilbene derivative diiso-thiocyanato-stilbene-disulphonic acid (DIDS) and the diphenylamine-2-carboxylate analogue 5-nitro-2-(3-phenylpropylamino)-benzoate (NPPB) caused a dose-dependent, reversible flicker block of the small conductance and significantly reduced the macroscopic current flow through the channel. 6. In quiescent outside-out patches, when the pipette contained a 140 mM-CsCl solution with 10(-6) M-CaCl2, 1.2 mM-MgCl2 and 1 mM-GTP, and the bath contained Ringer solution, addition of the NK-1 receptor antagonists substance P methylester resulted in activation of the full conductance state and of smaller substates.(ABSTRACT TRUNCATED AT 400 WORDS)