Effect of 18beta-glycyrrhetinic acid on electromechanical coupling in the guinea-pig renal pelvis and ureter

Abstract

We have tested the effect of the gap junction inhibitor, 18beta-glycyrrhetinic acid (18betaGA) on electromechanical coupling in the guinea-pig renal pelvis and ureter by the sucrose gap technique. In the ureter 18betaGA (3 - 30 microM) produced a concentration-dependent inhibition of the spike component of the action potential (AP) and reduced contraction evoked by electrical stimulation. Neurokinin A (NKA) produced a slow depolarization with superimposed APs and phasic contractions of the ureter. 18betaGA (30 microM) markedly inhibited the depolarization and APs evoked by NKA. However the contractile response was more sustained in the presence than in the absence of 18betaGA. At 100 microM, 18betaGA inhibited the mechanical responses to NKA. KCl (80 mM) produced APs and phasic contractions followed by sustained depolarization and tonic contraction. At 30 microM 18betaGA markedly inhibited the KCl-evoked APs and phasic contractions without affecting the sustained responses. At 100 microM 18betaGA inhibited the tonic contraction to KCl. In the renal pelvis 18betaGA (30 microM) inhibited the amplitude of pacemaker potentials and accompanying contractions and induced the appearance of low-amplitude APs not associated with contraction. We conclude that, up to 30 microM, the action of 18betaGA is consistent with an inhibition of cell-to-cell electrical coupling via gap junctions. The single-unit character of smooth muscles in the guinea-pig upper urinary tract is partly converted to a multi-unit pattern. At high concentrations 18betaGA possesses non specific effects which limit its usefulness as a tool for studying the role of gap junctions in smooth muscles. British Journal of Pharmacology (2000) 129, 163 - 169

Figure 1

(A) tracing illustrating the time-dependent effect of 18βGA (30 μM) on the EFS-evoked AP of the guinea-pig ureter. Note that 18βGA abolished the spike component of the AP and reduced contraction. At 30 min from start of superfusion with 18βGA, a bell-shaped AP with long latency was recorded in the presence of 18βGA. Nifedipine 1 μM promptly aboplished the residual electrical and mechanical responses to EFS receorded in the presence of 18βGA. (B) shows the electrical (a) and mechanical (b) responses to EFS measured before and 30 min after addition of 18βGA on an expanded time-scale: note that, in the presence of 18βGA the contractile response to EFS ensued before an electrical event was detectable at this time (see discussion). Calibration bars in A also apply to B.

Figure 2

Concentration- and time-dependency of the effect of 18βGA on amplitude (A), latency (B), slope (C) of action potential (AP) and accompanying contraction induced by EFS in the guinea-pig ureter. Each value is mean±s.e.mean of 5–7 experiments. ^*Significantly different from control P<0.05.

Figure 3

Tracing illustrating the effect of 18βGA (30 μM for 30 min) on the electrical and mechanical responses induced by NKA (3 μM for 15 s) in the guinea-pig ureter. In control conditions NKA induced a slow depolarization with superimposed action potentials and phasic contractions. In the presence of 18βGA the electrical response to NKA were markedly diminished although a number of low amplitude action potentials were evoked; these were associated with low amplitude phasic contractions which fused together to produce a sustained tonic type contraction of the ureter smooth muscle.

Figure 4

Tracing illustrating the effect of 18βGA (30 μM for 30 min) on spontaneous electrical and mechanical activity of the guinea-pig proximal renal pelvis. Note that 18βGA induced the apperance of low amplitude action potentials not associated with contractions (marked by asterisks) while concomitantly decreasing the amplitude of action potentials and associated contractions.

Figure 5

Time course of the effect of 18βGA (30 μM) on frequency of spontaneous action potentials accompanied by contractions (A), frequency of AP (whether or not accompanied by contractions (B), amplitude of AP associated with contraction (C) and amplitude of contraction (D) of the guinea-pig proximal renal pelvis. Each value is mean±s.e.m. of 6–11 experiments. ^*Significantly different from control P<0.05.