High affinity regulation of cardiac Cl– and Ca2+ conductances by (13R)-spiroforskolin

Abstract

The effects of a new forskolin derivative, (13R)-spiroforskolin, on the ventricular cAMP-activated chloride current (I_Cl(cAMP)) and the atrial L-type calcium current (I_Ca,L) were measured by means of whole-cell recording from isolated guinea-pig cardiac myocytes at 30°C and 20–22°C, respectively. In contrast to forskolin, the derivative contains a tetrahydrofuran rather than a tetrahydropyran moiety. (13R)-spiroforskolin activated I_Cl(cAMP) in 58% of the ventricular myocytes studied. The concentration required for the half maximal effect (EC₅₀ value) amounted to 9.6×10^–11 M and was lower than the EC₅₀ value for forskolin (2.4×10^–8 M). (13R)-spiroforskolin evoked a smaller maximal I_Cl(cAMP) amplitude than forskolin. The rundown of the (13R)-spiroforskolin-activated I_Cl(cAMP) was faster than that of the forskolin-induced current. Neither forskolin nor (13R)-spiroforskolin in maximally effective concentrations increased I_Cl(cAMP) in cells containing high concentrations of cAMP. Furthermore, as an activator of atrial I_Ca,L (13R)-spiroforskolin displayed a smaller activation and a lower EC₅₀ value (5.8×10^–10 M) than forskolin (EC₅₀ value: 3.7×10^–7 M). The effect of (13R)-spiroforskolin was observed in only 30% of the atrial cells studied. None of the drugs exerted a stimulatory effect in atrial cells containing a high [cAMP]. The washout of the drug effect was significantly faster in (13R)-spiroforskolin- than in forskolin-treated atrial myocytes. We conclude that (13R)-spiroforskolin as a forskolin derivative displays unique characteristics. It is a more potent but less efficacious activator of cardiac ionic conductances than the parent compound. The results suggest that (13R)-spiroforskolin, like forskolin, most probably exerts its effects via stimulation of the adenylyl cyclase.