Ionic currents in single smooth muscle cells from the ureter of the guinea‐pig.

Abstract

SUMMARY1. Ionic currents underlying the action potential were recorded from enzymatically isolated smooth muscle cells of guinea-pig ureter.2. The action potential recorded from a single cell was similar to that from a multicellular preparation. It showed repetitive spikes on a plateau potential which followed the first spike. Treatment with 10 mM-tetraethylammonium (TEA) increased the amplitude and duration of the plateau phase and abolished the repetitive spikes.3. Under voltage clamp mode, at least two (maybe three) kinds of outward currents were activated during depolarizing pulses. The main outward current was Ca2"-dependent K+ current (IK(ca)), which was mostly blocked in Ca2`-free solution, or by application of 1 mM-cadmium (Cd2+) or 2 mM-tetraethylammonium (TEA).IK(ca) was greatly decreased by treatment with 5 mM-caffeine or an addition of 10 mM-EGTA in a pipette solution.4. In the presence of 1 mM-Cd2' and 2 mM-TEA, a small transient outward current remained. 4-Aminopyridine (1 mM) suppressed the transient outward current by about 40 %. Time-and voltage-dependent delayed rectifier outward currents were small in ureter cells. An inwardly rectifying K+ current was not detected.5. An application of 1 mM-Cd2+, 5 mM-cobalt (Co2+), 1 mM-lanthanum (La3+) or 0-1 IuM-nifedipine completely blocked the action potential. Replacement of 80-90 % of extracellular Na+ with Li+ or Tris almost abolished the plateau potential and repetitive spikes but did not change significantly the first spike. 6. In the presence of 30 mM-TEA, the inward current elicited by depolarization was monophasic and lasted for more than 1 s. Application of 1 mM-Cd2+, 1 mM-La3 , 0-1 /LM-nifedipine, or 5 mM-Co2+ completely blocked inward current. The replacement (87 %) of extracellular Na+ ions with Li+, Tris, sucrose or TEA speeded up the decay of inward current; the inward current decreased by 10-60 % at the end of a 500 ms pulse.7. Even in low-Na+ solution (120 mM-TEA), the inactivation of ICa had a quite slow component (T = 1 S), in addition to another faster component (T = 100 ms) at 0 mV. When short depolarizing clamp pulses (50 ms) were repetitively applied at short intervals (50 ms) and with interpulse voltage of -10 or -20 mV to mimic the repetitive spikes on the plateau of the action potential, the decline of peak Ca2+ 5-2 Y. IMAIZUMI, K. MURAKI AND M. WATANABE current during the train of pulses was smaller than the decay of Ca2+ current during a long pulse.8. These results indicate that (1) the repetitive spikes on the plateau phase of the action potential are attributable to the slow inactivation and rapid reactivation of I4a and repetitive activation of IK(Ca) due to Ca2+-induced Ca2+ release from stores, and (2) the long plateau phase in the action potential may be due to a slowly inactivating Iba, a Na+-dependent late inward current, and a small delayed rectifier outward current.

Ca2+ images and K+ current during depolarization in smooth muscle cells of the guinea‐pig vas deferens and urinary bladder

“…Smooth muscle cells were isolated from vas deferens and urinary bladder using a slight modification of a previously described method (Imaizumi, Muraki & Watanabe, 1989). In brief, male guinea-pigs were stunned by a blow to the head and killed by exsanguination.…”

Pacemaker activity in the upper urinary tract

“…After reaching the peak of its action potential, the membrane maintains a depolarized state for a period of time (plateau of the action potential) before the transmembrane potential returns to its resting level (repolarization). The plateau phase depends on the persistence of an inward Ca 2+ current, Na + influx through a voltage-dependent Na + channel, an inward calcium dependent chloride current (ICl(Ca)), in the rat but not in the guinea pig ureter, and the weakness of an outward voltage-gated and Ca 2+ -activated K + current (Kuriyama and Tomita, 1970;Imaizumi et al, 1989;Smith et al, 2002). In the guinea pig there are oscillations on the plateau of the action potential which appear to depend on the repetitive activation of an inward Ca 2+ current and of an outward Ca 2+ -dependent K + current (Kuriyama and Tomita, 1970;Imaizumi et al, 1989).…”

“…The plateau phase depends on the persistence of an inward Ca 2+ current, Na + influx through a voltage-dependent Na + channel, an inward calcium dependent chloride current (ICl(Ca)), in the rat but not in the guinea pig ureter, and the weakness of an outward voltage-gated and Ca 2+ -activated K + current (Kuriyama and Tomita, 1970;Imaizumi et al, 1989;Smith et al, 2002). In the guinea pig there are oscillations on the plateau of the action potential which appear to depend on the repetitive activation of an inward Ca 2+ current and of an outward Ca 2+ -dependent K + current (Kuriyama and Tomita, 1970;Imaizumi et al, 1989). Repolarization is due to activation of an outward Ca 2+ -dependent K + current (IK(Ca)) which is sensitive to inhibition by tetraethylammonium (TEA) and charbydotoxin and of a voltage dependent Ca 2+ -insensitive outward K + current (ITO) that is TEA insensitive and 4-aminopyridine (4-AP) sensitive (Imaizumi et al, 1989(Imaizumi et al, , 1990Lang, 1989;Sui and Kao, 1997c).…”

“…In the guinea pig there are oscillations on the plateau of the action potential which appear to depend on the repetitive activation of an inward Ca 2+ current and of an outward Ca 2+ -dependent K + current (Kuriyama and Tomita, 1970;Imaizumi et al, 1989). Repolarization is due to activation of an outward Ca 2+ -dependent K + current (IK(Ca)) which is sensitive to inhibition by tetraethylammonium (TEA) and charbydotoxin and of a voltage dependent Ca 2+ -insensitive outward K + current (ITO) that is TEA insensitive and 4-aminopyridine (4-AP) sensitive (Imaizumi et al, 1989(Imaizumi et al, , 1990Lang, 1989;Sui and Kao, 1997c).…”

“…Thus, cells at the UVJ of the dog may act as pacemaker cells when freed of control from the primary proximally located pacemaker. Latent pacemaker cells are present in all regions of the ureter (Imaizumi et al, 1989;Meini et al, 1995). (Fig.…”