Possible mechanisms underlying the vasodilatation induced by olprinone, a phosphodiesterase III inhibitor, in rabbit coronary artery

Abstract

The possible mechanisms underlying the vasodilatation induced by olprinone, a phosphodiesterase type III inhibitor, were investigated in smooth muscle of the rabbit coronary artery. Isometric force and membrane potential were measured simultaneously using endothelium-denuded smooth muscle strips. Acetylcholine (ACh, 3 microM) produced a contraction with a membrane depolarization (15. 2+/-1.1 mV). In a solution containing 5.9 mM K(+), olprinone (100 microM) hyperpolarized the resting membrane and (i) caused the absolute membrane potential level reached with ACh to be more negative (but did not reduce the delta membrane potential seen with ACh, 15.2+/-1.8 mV) and (ii) attenuated the ACh-induced contraction. In a solution containing 30 mM K(+), these effects were not seen with olprinone. Glibenclamide (10 microM) blocked the olprinone-induced membrane hyperpolarization. 4-AP (0.1 mM) significantly attenuated the olprinone-induced resting membrane hyperpolarization but TEA (1 mM) had no such effect. Glibenclamide (10 +microM), TEA (1 mM) and 4-AP (0.1 mM), given separately, all failed to modify the inhibitory actions of olprinone on (i) the absolute membrane potential level seen with ACh and (ii) the ACh-induced contraction. It is suggested that olprinone inhibits the ACh-induced contraction through an effect on the absolute level of membrane potential achieved with ACh in smooth muscle of the rabbit coronary artery. It is also suggested that glibenclamide-sensitive, ATP-sensitive K(+) channels do not play an important role in the olprinone-induced inhibition of the ACh-induced contraction.

Figure 1

Effects of olprinone (100 μM) on ACh (3 μM)-induced membrane depolarization and contraction in the presence and absence of glibenclamide (10 μM) in endothelium-denuded rabbit coronary artery. (A) shows actual tracings of simultaneously recorded membrane potential (upper traces) and isometric force (lower traces) in control (A,a), in the presence of olprinone (A,b) and in the presence of both olprinone and glibenclamide (A,c). Recordings were all from the same cell. (B) Effect of glibenclamide (10 μM) on membrane hyperpolarization induced by olprinone (100 μM).

Figure 2

(A) Effects of olprinone (10 and 100 μM) on membrane potential in the presence of 3 μM ACh and in the quiescent state in endothelium-denuded rabbit coronary arteries. Each data point shows mean from four preparations with s.e.mean shown by vertical lines. (B) Effects of olprinone (10 and 100 μM) on ACh (3 μM)-induced contraction in endothelium-denuded rabbit coronary arteries. Each data point shows mean from four preparations with s.e.mean shown by vertical lines.

Figure 3

Effects of olprinone on ACh (3 μM)-induced depolarization and contraction in a solution containing either 5.9 mM K⁺ (normal K⁺ concentration in Krebs solution) or 30 mM K⁺. (A) shows membrane potential in the absence or presence of 3 μM ACh. Control-1 and Control-2 represent values obtained in the presence of 5.9 mM K⁺ or 30 mM K⁺, respectively, but without olprinone. (B) shows relative force evoked by ACh (3 μM). The maximum amplitude of contraction induced by 3 μM ACh in a solution containing 5.9 mM K⁺ was normalized as 1.0 (control-1). ^**P<0.01.

Figure 4

Effects of glibenclamide (10 μM), 4-AP (1 mM) and TEA (1 mM) on the inhibitory action of olprinone on the effect of ACh on membrane potential and contraction. (A) Membrane potential recorded in the absence or presence of ACh with or without olprinone and with or without the K⁺ channel blockers. Control-1 represents the control data for the effects of olprinone (with or without glibenclamide), the series of responses being evoked in one and the same strip. Control-2 represents control data for the effect of olprinone (with or without 4-AP). Control-3 represents control data for the effect of olprinone (with or without TEA). (B) ACh (3 μM)-induced contractions expressed as relative force. Data are from the same three series of tests as those shown in (A). Each column shows the mean from 4–8 preparations with s.e.mean shown by vertical lines. ^*P<0.05.

Figure 5

Relationship between membrane potential (E_m) and relative force (F_r) in coronary arteries contracted with 3 μM ACh in the presence of olprinone (1–100 μM) or isoprenaline (0.005–0.1 μM). The maximum amplitude of contraction induced by 3 μM ACh with no olprinone and no isoprenaline was normalized as 1.0. Relative force (F_r) was calculated as log₁₀F_r to correct for the non-linear relationship between E_m and F_r.

Figure 6

Effects of low Na⁺ solution (15.5 mM), NPPB (10 μM) and Co²⁺ (2 mM) on ACh (3 μM)-induced membrane depolarization (A) and contraction (B) in endothelium-denuded rabbit coronary artery. (A) Membrane potential recorded in the absence or presence of ACh in strips treated or not treated with the various agents. Control-1 represents the control data for the effects of low Na⁺ solution, the series of responses being evoked in one and the same strip. Control-2 represents control data for NPPB and control-3 that for Co²⁺. (B) ACh (3 μM)-induced contractions expressed as relative force. Each column shows the mean from five preparations with s.e.mean shown by vertical lines. ^*P<0.05 vs the corresponding control (‘control' being a relative force of 1.0 in B).