Phorbol 12,13-dibutyrate produces negative inotropism and selective antagonism of responses to adrenoceptor agonists in rat atria

Mechanism of the negative inotropic effects of alpha 1-adrenoceptor agonists on mouse myocardium

This study was done to identify the mechanism of the alpha1-adrenoceptor (AR) mediated negative inotropic effects of phenylephrine (PE) on adult mouse myocardium. As reported by others, we also found that the nonselective alpha1AR agonist PE produced a negative inotropic effect on ventricular strips from adult mice that was inhibited by the alpha1AAR antagonist 5-methylurapidil (5MU) but not by the alpha1BAR antagonist chloroethylclonidine (CEC) or the alpha1DAR antagonist BMY 7378. The selective alpha1AAR agonist A61603 also produced a negative inotropic effect, which was antagonized by 5MU. Phorbol 12,13-dibutyrate (activator of all PKC isoforms) mimicked the negative inotropic responses to PE and A61603. The negative inotropic effects of PE were inhibited by bisindolylmaleimide (inhibitor of all PKC isoforms) but not by Gö 6976 (inhibitor of Ca2+-dependent PKC). Rottlerin, an inhibitor of Ca2+-independent PKCdelta, antagonized the negative inotropic effects of PE and A61603. PE and A61603 increased the translocation of PKCdelta, which was prevented by rottlerin. These data suggest that the alpha1AR-mediated negative inotropy on adult mouse myocardium is signaled by Ca2+-independent PKCdelta.

Cardiovascular α1-adrenoceptor subtypes: functions and signaling

α1-Adrenoceptors (α1AR) are G protein-coupled receptors and include α1A, α1B, and α1D subtypes corresponding to cloned α1a, α1b, and α1d, respectively. α1AR mediate several cardiovascular actions of sympathomimetic amines such as vasoconstriction and cardiac inotropy, hypertrophy, metabolism, and remodeling. α1AR subtypes are products of separate genes and differ in structure, G protein-coupling, tissue distribution, signaling, regulation, and functions. Both α1AAR and α1BAR mediate positive inotropic responses. On the other hand, cardiac hypertrophy is primarily mediated by α1AAR. The only demonstrated major function of α1DAR is vasoconstriction. α1AR are coupled to phospholipase C, phospholipase D, and phospholipase A2; they increase intracellular Ca2+ and myofibrillar sensitivity to Ca2+ and cause translocation of specific phosphokinase C isoforms to the particulate fraction. Cardiac hypertrophic responses to α1AR agonists might involve activation of phosphokinase C and mitogen-activated protein kinase via Gq. α1AR subtypes might interact with each other and with other receptors and signaling mechanisms.Key words: cardiac hypertrophy, inotropic responses, central α1-adrenoreceptors, arrythmias.

Characterization of calcium-dependent forms of protein kinase C in adult rat ventricular myocytes

The presence and subcellular localization of the Ca2+-dependent protein kinase C (PKC) isoforms alpha and beta were investigated in freshly isolated adult rat cardiac ventricular myocytes. PKC activity was measured in cytosolic and particulate fractions prepared from control myocytes and those treated with either phorbol ester (phorbol 12-myristate 13-acetate, PMA) or a permeant synthetic diacylglycerol analog (1-oleoyl-2-acetylglycerol, OAG) in the absence or presence of an inhibitor of diacylglycerol kinase activity, compound R59022. Preliminary studies detected no Ca2+-/phospholipid-dependent histone kinase activity in either subcellular fraction. To reproducibly observe Ca2+-/phospholipid-dependent protein kinase activity, partial purification using a MonoQ HR 5/5 column and the presence of the peptide inhibitor of the cAMP-dependent protein kinase were essential. MonoQ chromatography of cytosolic and particulate fractions resulted in three peaks of Ca2+/phospholipid-dependent protein kinase activity. In the cytosolic fraction a large peak of activity eluted at 230-300 mM NaCl. Isoform-specific antisera indicated both PKC alpha and PKC beta were present. In the particulate fraction two peaks of Ca2+-/phospholipid-dependent protein kinase activity, both containing PKCa immunoreactivity, were observed. The larger peak eluted at 230-300 mM NaCl. In addition, a peak eluting at lower salt concentrations contained a Ca2+-/phospholipid-independent histone kinase activity. This peak of kinase activity contained PKC alpha immunoreactive bands of 80- and 50-kDa. The 80-kDa band was the holoenzyme of PKC alpha whereas the band of lower molecular mass was likely a proteolytic fragment. In both cytosolic and particulate fractions, the peak of kinase activity eluting at 230-300 mM NaCl contained PKC alpha in the form of an 80-kDa doublet; this suggested the presence of autophosphorylated PKC. Incubation of the myocytes with PMA, but not OAG, resulted in translocation of PKC from the cytosolic to the particulate fraction. Curiously, a transient decrease in PKC activity was observed in both subcellular fractions following treatment with either OAG or ethanol (1%). Results from this study show that freshly isolated adult rat cardiac ventricular myocytes contain both PKC alpha and PKC beta, and that these isoforms translocate to the particulate fraction in response to treatment with PMA, but not OAG.

Influence of phorbol esters on contractile force, action potential and calcium current of isolated guinea-pig heart tissues

Phorbol-12, 13-dibutyrate (PDB) reduced concentration-dependently the contractile force of guinea-pig papillary muscles (EC50 1.07 mumol/l) while phorbol-12-myristate-13-acetate (PMA) was ineffective. The protein kinase C inhibitors staurosporine (0.1 mumol/l) and polymyxin B (70 mumol/l) did not antagonize the negative inotropic effect of PDB. Neither PMA nor PDB, in concentrations up to 30 mumol/l, caused significant changes of the maximum depolarization velocity, the action potential duration or the functional refractory period in intact papillary muscles. In isolated ventricular cardiomyocytes the inward calcium current was halved by either 1 mumol/l PDB or 10 mumol/1 PMA. PKC inhibitors attenuated, but could not completely abolish this effect of the phorboles. It is concluded that the negative inotropic effect of PDB is caused by a reduction of the slow inward calcium current and that this inhibition is, for the greater part, not mediated by an activation of protein kinase C.

Effect of pre-treating rat atria with potassium channel blocking drugs on the electrical and mechanical responses to phenylephrine

Action potential duration (APD) and systolic developed tension (SDT) were recorded from electrically paced rat atria before, during and after treatment with phenylephrine in vitro. Phenylephrine caused a prolongation of APD and an increase in SDT, the magnitude of which depended upon the frequency of pacing of the atria. Effects very similar to those elicited by phenylephrine were produced by exposing the atria to 4-aminopyridine, 3,4-diaminopyridine, CsCl, amantadine or sparteine. Atria that had been pre-treated with any of the latter five compounds showed a diminished responsiveness when subsequently treated with maximally effective concentrations of phenylephrine, and vice versa. These findings are consistent with blockade of potassium ion channels being responsible for the observed responses to all of the above agents.