Different patterns of protein kinase C redistribution mediated by alpha 1-adrenoceptor stimulation and phorbol ester in rat isolated left ventricular papillary muscle

alpha1-Adrenoceptor-mediated Negative Inotropic Effect Caused by Intracellular Ionic Activities in Guinea-pig Papillary Muscle

In guinea-pig papillary muscle, phenylephrine (PE), an agonist of alpha1-adrenoceptor (alpha1-AR), led to a transient negative inotropic effect (NIE) and a subsequent sustained positive inotropic effect (PIE). To clarify the ionic mechanisms underlying the NIE, we measured the [Na+]i or [pH]i by ion-selective microelectrodes. PE produced a decrease in the intracellular Na+ concentration ([Na+]i) and an increase in intracellular pH ([pH]i). During the phase of NIE, PE produced only a (-) change of [Na+]i (Delta[Na+]i). With a decrease in extracellular Na+ or an increase in extracellular Ca2+, the PE-induced NIE was attenuated and PE produced (+)Delta[Na+]i. The PE-induced NIE and (-)Delta[Na+]i were definitely strengthened by lowering the bath temperature or increasing the stimulation frequency. 2-(2,6-di-methoxyphenoxyethyl)amino-methyl-1,4-benzidioxane HCl, an antagonist of alpha1A-AR, completely abolished the PE-induced NIE and (-)Delta[Na+]i. Phorbol 12,13-dibutyrate, an activator of protein kinase C (PKC), decreased the baseline [Na+]i and twitch force and increased the baseline [pH]i in mimicry of PE. Pretreatment with 1-5(isoquinolinesulphonyl)-2-methylpiperazine, an inhibitor of PKC, abolished the PE-induced NIE and (-)Delta[Na+]i. During pretreatment with benzamil, an inhibitor of Na+/Ca2+ exchange, we found that the PE-induced NIE and (-)Delta[Na+]i were reversibly abolished. Our results indicate the PE-induced NIE may be elicited upon the activation of Na+/Ca2+ exchange which can be attributed to the (-)Delta[Na+]i. (-)Delta[Na+]i is mediated through a PKC-dependent pathway via an activation of alpha1A-AR subtype and its effect could be strengthened remarkably at high [Na+]i and [Ca2+]i values.

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.

Expression of alpha1-adrenergic receptor subtypes in heart cell culture

Three alpha1-AR subtypes have been cloned so far and are designated as alpha1a, alpha1b,, and alpha1d. Organ-specific distribution pattern and subtype-specific effects are known but not fully understood. To address a cell-type specific expression pattern in the heart we investigated expression pattern of alpha1-AR subtypes on RNA- and protein-level in heart tissue, cultured cardiomyocytes and non-myocytes of the rat. Each alpha1AR-subtype mRNA was present in neonatal and adult rat heart culture but the relative distribution pattern was significantly different. While the alpha1a-AR subtype is preferentially expressed in adult cardiomyocytes, the alpha1b-AR subtype was preferentially expressed in the non-myocyte cell fraction. The RT-PCR results were confirmed by Western-blotting (alpha1b) and immunocytochemical studies. Incubation with an alpha1-agonist (phenylephrine) for 72 h led to a significant reduction of the alpha1b-AR in neonatal heart cell culture on both mRNA and protein level. In contrast, incubation with an alpha1-antagonist (prazosin) induced a 1.6 fold upregulation of the alpha1a-AR mRNA without significant effects on radioligand binding and functional assay. The results indicate a distribution pattern of the alpha1-AR subtype which is specific for cell type and ontogeny of the rat heart and may be regulated by adrenergic agents.

Pertussis toxin-sensitive and -insensitive mechanisms of alpha1-adrenoceptor-mediated inotropic responses in rat heart

In rat left ventricular papillary muscle, phenylephrine, an alpha1-adrenoceptor agonist, induced a triphasic inotropic response; an initial transient, small, positive inotropic effect followed by a transient chloroethylclonidine-sensitive negative inotropic effect and a sustained 2-(2,6-dimethoxyphenoxyethyl)aminomethyl-1,4-benzodioxane (WB4101)-sensitive positive inotropic effect. Treatment with pertussis toxin for 2 days significantly inhibited only the transient negative inotropic effect without changing the sustained positive inotropic effect. This treatment also prevented the acetylcholine (1 microM)-induced negative inotropic effect. Further, phenylephrine-induced transient negative inotropic effect was attenuated in the presence of ouabain. These results suggest that pertussis toxin-sensitive or -insensitive G-protein may be responsible for alpha1-adrenoceptor subtype-mediated negative inotropic effect or positive inotropic effect, respectively, in which the transient negative inotropic effect was produced via the stimulation of Na+, K+ pump, presumably through pertussis toxin-sensitive G-protein-dependent pathway.

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.

Positive inotropy mediated by diacylglycerol in rat ventricular myocytes

Many neurohormones stimulate phospholipid hydrolysis and elevate diacylglycerol in the mammalian heart, but the physiological consequences of these intracellular events are unclear. Regulation of myocardial contraction by diacylglycerol was investigated in the present study by releasing the diacylglycerol analogue dioctanoylglycerol (diC8) within adult rat ventricular myocytes by using a light-sensitive caged compound. This approach permitted us to avoid exposure of myocytes to extracellular diC8 and yet to control the amount of diC8 released into the cells. Photorelease of diC8 produced a slowly developing (half-time, 1.9 +/- 0.1 minute; n = 26) but robust (406 +/- 42%) enhancement of twitch amplitude in electrically paced myocytes (0.5 Hz, 1 mmol/L Ca2+, Ringer's solution [pH 7.4], 22 degrees C). This positive inotropic effect was dose dependent, stereospecific for the S-enantiomer of diC8, synergistically enhanced by arachidonic acid, and blocked by the protein kinase C inhibitor chelerythrine. The data provide evidence that diacylglycerol can induce a strong positive inotropic effect in mammalian ventricular muscle, possibly by activating protein kinase C. By contrast, perfusion of diC8 extracellularly onto myocytes caused a 42 +/- 2% decline in twitch amplitude, in accordance with previous reports. To account for this dependence on how diC8 is applied, we postulate that diC8 has distinct physiological actions at intracellular and extracellular sites. The peptide neurohormone endothelin-1, which elevates diacylglycerol in cardiac tissues, produced a positive inotropic effect that was similar to the response to photoreleased diC8. The diacylglycerol/protein kinase C pathway has now become a good candidate for mediator of at least a component of the positive inotropy associated with agents that stimulate phospholipid turnover in adult mammalian myocardium.

alpha 1-adrenoceptor-mediated negative inotropy of adrenaline in rat myocardium

1. The effect of alpha- and beta-adrenoceptor stimulation on isotonic contraction was investigated on right ventricular papillary muscles of the rat, stimulated at a rate of 0.5 Hz. 2. Adrenaline (0.5 microM) induced a slight but significant negative inotropic effect: shortening decreased from 0.137 +/- 0.058 to 0.122 +/- 0.059 muscle lengths (mean +/- S.D.; -11%, P < 0.0001) and maximum shortening velocity from 2.9 +/- 1.2 to 2.7 +/- 1.3 muscle lengths s-1 (-7%, P < 0.025). 3. The negative inotropic effect of adrenaline was enhanced after blocking the beta-adrenoceptors with 50 microM atenolol. On the other hand, exposure to adrenaline after blocking the alpha-adrenoceptors with 50 microM phentolamine resulted in an increase in shortening as well as in maximum shortening velocity. 4. Stimulation of the beta-adrenoceptors with 0.5 microM isoprenaline caused marked positive inotropic effects, whereas stimulation of the alpha 1-adrenoceptors with 0.5 microM phenylephrine regularly resulted in a long-lasting decrease in shortening and maximum shortening velocity. 5. 1,2-Dioctanoyl-sn-glycerol (1,2-DOG) and adrenaline induced an activation of protein kinase C (PKC) with translocation of this enzyme from the cytosol to the sarcolemma. 6. Activation of PKC with 10 microM 1,2-DOG and 0.5 microM adrenaline was accompanied by a decrease in shortening and maximum shortening velocity. Inhibition of PKC with 0.1 microM staurosporine abolished the negative inotropic effect of adrenaline. 7. From these results we conclude that a low dose of adrenaline stimulates not only beta-but also alpha-adrenoceptors and that the observed negative inotropic effect of adrenaline is mediated by alpha 1-adrenoceptors, linked to the diacylglycerol-PKC signal transduction pathway.

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.

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

Rat atria loaded in vitro with the dye INDO-1 produced fluorescence signals indicative of changes in cytoplasmic calcium ion concentration ([Ca2+]c). Such atria showed systolic/diastolic fluctuations indicative of a systolic rise and a diastolic fall in both tension and [Ca2+]c. Positively inotropic responses of the atria to isoprenaline, phenylephrine, ouabain or 4-aminopyridine were associated with fluorescence changes indicative of increased systolic increments in [Ca2+]c. Treatment of atria with phorbol dibutyrate, on the other hand, produced negative inotropism and fluorescence changes indicative of declining systolic increments in [Ca2+]c. Pretreating atria with phorbol dibutyrate diminished both the inotropic and fluorescence responses to subsequent treatment with phenylephrine or isoprenaline, although responses to ouabain or 4-aminopyridine were unchanged. Exposure of atria to a selective inhibitor of protein kinase C mitigated the effects of pretreatment with phorbol dibutyrate, but failed to modify responses to phenylephrine or isoprenaline that were produced in the absence of phorbol dibutyrate.