Sustained Diacylglycerol Formation in Norepinephrine-Stimulated Rat Heart is Associated with α1-Adrenergic Receptor

The whole‐cell patch clamp was employed to study Na+‐K+ pump current (Ip) in acutely isolated myocytes. α‐Adrenergic receptors were activated with noradrenaline (NA) after blocking β‐adrenergic receptors with propranolol. Ip was measured as the current blocked by strophanthidin (Str). Activation of α‐receptors by NA increased Ip in a concentration‐dependent manner. The K0.5 depended on intracellular calcium ([Ca2+]i), however maximal stimulation did not. At 15 nm[Ca2+]i the K0.5 was 219 nm NA whereas at 1.4 μm [Ca2+]i it was 3 nm. The voltage dependence of Ip was not shifted by NA at either high or low [Ca2+]i. At each voltage, maximal stimulation of Ip was 14‐15 %. Staurosporine (St), an inhibitor of protein kinase C (PKC), eliminated the α‐receptor‐mediated stimulation of Ip at either high or low[Ca2+]i. The stimulation of Ip was independent of changes in intracellular sodium or external potassium concentrations, and did not reflect a change in affinity for Str. Phenylephrine, methoxamine and metaraminol, three selective α1‐adrenergic agonists, stimulate Ip in a similar manner to NA. Stimulation of Ip by NA was eliminated by prazosin, an α1‐antagonist, but was unaffected by yohimbine, an α2‐antagonist. We conclude noradrenaline activates ventricular α1‐receptors, which are specifically coupled via PKC to increase Na+‐K+ pump current. The sensitivity of the coupling is [Ca2+]i dependent, however the maximal increase in pump current is [Ca2+]i and voltage independent.

Section: Discussionmentioning

confidence: 99%

α‐Adrenergic effects on Na⁺‐K⁺ pump current in guinea‐pig ventricular myocytes

Wang

Gao

Mathias

et al. 1998

“…Diacylglycerol (Okumura et al 1988) and IP3 levels (Poggioli et al 1986;Otani et al 1988;Scholz et al 1988) are increased in the heart in response to al-adrenoceptor stimulation. Direct effects of IP3 on membrane conductances have not been shown.…”

Section: U Jahnel and Othersmentioning

confidence: 99%

“…In the heart, ax-adrenoceptor stimulation increases the levels of several metabolites of phosphatidyl inositol turnover (Poggioli, Sulpice & Vassort, 1986; Otani, Otani & Das, 1988;Scholz, Schaefer, Schmitz, Scholz, Steinfath, Lohse, Schwabe & Puurunen, 1988;Okumura, Kawai, Hashimoto, Ito, Ogawa & Satake, 1988). However, the precise role of DAG and/or IP3 for the positive inotropic effect of al-adrenoceptor stimulation has remained unclear.…”

Section: Introductionmentioning

confidence: 99%

Depolarization‐induced influx of sodium in response to phenylephrine in rat atrial heart muscle.

Jahnel

Nawrath

Carmeliet

et al. 1991

SUMMARY1. The effects of az-adrenoceptor stimulation on transmembrane potential, currents and ion fluxes were investigated in multicellular preparations and/or single cells obtained from the left atrium of rat hearts.2. In multicellular preparations, phenylephrine caused a concentration-dependent positive inotropic effect, an increase in action potential duration, and a decrease in resting potential; the effects were antagonized by phentolamine.3. In the presence of phenylephrine (100 ,umol/l), two levels of resting potential were observed when the preparations were, alternately, electrically stimulated or kept at rest (-74 +1 mV during activity and -62 + 4 mV at rest; means + S.E.M.; n = 9).4. In resting preparations, the depolarization in response to phenylephrine was eliminated in low-Na+ solution (12 mmol/l) and antagonized by tetrodotoxin (10 ,tmol/l).5. The phenylephrine-induced depolarization was also seen in nominally Ca2+-free solution and in the presence of (-)-devapamil (1 psmol/l). 11. In conclusion, we presume the following sequence of events in response to phenylephrine in rat atrial heart muscle. First, the stimulation of a,-adrenoceptors t To whom reprint requests and correspondence should be addressed.MS 7852

“…It is known that a-adrenoceptor stimulation results in inositol phospholipid breakdown in mammalian heart (Woodcock, McLeod, Smith & Clark, 1987). Diacylglycerol production is also enhanced (Okamura, Kawai, Hashimoto, Ito, Ogawa & Satake, 1988), which will result in activation of protein kinase C. If protein kinase C stimulates Na+-H+ exchange, as suggested above, a-adrenoceptor activation may result in an intracellular alkalinisation which could underlie the increase in force.…”

mentioning

confidence: 90%

Effects of phorbol ester on contraction, intracellular pH and intracellular Ca2+ in isolated mammalian ventricular myocytes.

MacLeod

Harding

1991

SUMMARY1. We have investigated the actions of certain phorbol esters on the intracellular pH, intracellular Ca2+ and contractility of isolated rat and guinea-pig cardiac myocytes. Intracellular pH was measured using 2',7'-bis(carboxyethyl)-5(6)-carboxyfluorescein (BCECF) and intracellular Ca2+ was measured using Fura-2.2. Application of the phorbol ester 12-0-tetradecanoylphorbol 13-acetate (also called phorbol 12-myristate 13-acetate) (TPA) (which activates protein kinase C) to rat cardiac myocytes significantly increased cell shortening by 116 + 34% (n = 8) (p < 002). The rate of change of cell length during contraction (i.e. + dL/dt) increased from 67-2 + 817 gim/s to 127-7 + 14-1 gm/s (n = 7). The rate of change of cell length during relaxation (-dL/dt) increased from 55-8 + 7X4 gm/s to 1 18-9 + 1241 gcm/s (n = 7). Time to peak shortening was unchanged.3. Application of 4a-phorbol 12,13-didecanoate, which does not activate protein kinase C, did not affect rat myocyte contractility. An insignificant decrease in contractility (by 7-5 + 7'5%) was observed (n = 5). The positive inotropic effect of TPA may therefore be evoked through an activation of protein kinase C.4. In rat myocytes we have measured the changes of pHi and contractility (cell shortening) during an alkalosis and acidosis induced by exposure to and subsequent removal of NH4Cl both in the presence and absence of TPA. Recovery times from an acid load were significantly (p < 005) enhanced by 1541+ 69 % (n = 13) in the presence of TPA. Recovery times of cell shortening were also more rapid (p < 005) by an average of 59-1 + 10-6 % (n = 5) in the presence of TPA. Recovery times were unchanged in the presence of 4-phorbol 12,13-didecanoate (wirhich does not activate protein kinase C).5. Since pHi recovery of an isolated myocyte from an acid load is partially inhibited by the presence of 1 mM-amiloride and inhibited by removing extracellular Na+ then it is suggested that, like pHi regulation in sheep heart Purkinje fibres, pH1 recovery in rat cardiac ventricular myocytes is mainly through sarcolemmal Na+-H+ exchange. We suggest that in the presence of TPA the Na+-H+ exchange is stimulated. (diastolic) [Ca21]. Fura-2 ratios increased by 21+4%(n = 5). We conclude that some of the positive inotropic effect produced by phorbol esters which activate protein kinase C is caused by an increase in systolic Ca2+.