ATP-induced stimulation of calcium binding to cardiac sarcolemma

GTP-independent stimulation of rabbit heart adenylate cyclase by isoproterenol at physiological ATP concentrations

Isoproterenol increased the activity of the adenylate cyclase of rabbit heart sarcolemmal membranes in the absence of added GTP. ATP, the ATP-regenerating system, and the sarcolemmal membrane preparation were eliminated as possible sources of contaminating GTP. Isoproterenol-stimulation increased as ATP was raised. At 0.5 mM ATP, isoproterenol increased activity by 19% whereas at 5 mM ATP isoproterenol increased activity by 121%. There was no change in basal activity between 0.5 and 5 mM ATP. Stimulation by Gpp(NH)p and NaF increased slightly between 0.5 and 5 mM ATP; stimulation by KCl was unaffected. GTP does not activate cyclase d. GTP does not activate cyclase to the same extent as Gpp(NH)p even though the two act at the same site on Ns (the stimulatory guanine nucleotide-binding protein). GTP decreased cyclase activation by Gpp(NH)p in a concentration-dependent fashion when the two were added to the assay simultaneously. Increasing ATP from 0.5 to 5 mM did not reduce activation by Gpp(NH)p when both were added simultaneously to the assay. This suggests that ATP does not interact with the same site as Gpp(NH)p. ATP gamma S, an analogue of ATP which irreversibly thiophosphorylates proteins, did not irreversibly support activation by isoproterenol. The effect of ATP in supporting isoproterenol stimulation is not, therefore, thought to be due to phosphorylation of a protein.

An antiadrenergic effect of adenosine on guinea-pig but not rabbit ventricles

The antiadrenergic effect of adenosine was investigated using isolated guinea-pig heart and guinea-pig and rabbit papillary muscle. Adenosine, 15 microM, completely abolished the increased tension stimulated by 0.1-1.0 nM isoprenaline in Langendorff-perfused guinea-pig hearts. With guinea-pig papillary muscles, adenosine decreased by 40% the increased force stimulated by 1-10 nM isoprenaline. When 5 microM 2-chloroadenosine was used in conjunction with 1 unit ml-1 adenosine deaminase, a complete inhibition of the isoprenaline-stimulated tension was seen in guinea-pig papillary muscles. The antiadrenergic effect of 2-chloroadenosine was blocked by 8-phenyltheophylline. In rabbit, there was little effect of 2-chloroadenosine (plus deaminase) on isoprenaline-stimulated tension. (-)-N6 (R-phenylisopropyl)-adenosine (PIA) had no effect on basal or isoprenaline-stimulated adenylate cyclase activity of guinea-pig or rabbit sarcolemmal membranes. We conclude that the antiadrenergic effect of adenosine is mediated by A type receptors and is seen in guinea-pig but not rabbit. Production of adenosine by superfused papillary muscle may obscure the effect of added adenosine. We find no evidence that the antiadrenergic effect is mediated by inhibition of adenylate cyclase.

Inhibition of rabbit cardiac adenylate cyclase by theophylline

Theophylline inhibits basal adenylate cyclase activity as well as cyclase stimulated by sodium chloride, sodium fluoride, GTP or 5'-guanylimidodiphosphate. This inhibition, is dose-dependent and shows non-competitive inhibition, with respect to MgATP. The presence of adenosine deaminase does not alter the effect of theophylline. The inhibition produced by theophylline is not additive with that due to 2'-deoxyadenosine 3'-monophosphate (a P-site agonist). It is suggested that theophylline may act at the P-site to reduce adenylate cyclase activity.

Cholesterol effect on enzyme activity of the sarcolemmal (Ca2+ + Mg2+)-ATPase from cardiac muscle

The effect of cholesterol incorporation and depletion of the cardiac sarcolemmal sacs on (Ca2+ + Mg2+)-ATPase activity was examined. Cholesterol incorporation to the sarcolemmal sacs was achieved utilizing an in vivo and an in vitro procedure. Cholesterol depleted membranes were obtained in vitro after incubation of the sarcolemmal sacs with inactivated plasma. Arrhenius plots of the (Ca2+ + Mg2+)-ATPase activity showed a triphasic curve when the assays were carried out using a temperature range between 0 and 40 degrees C. The sarcolemmal (Ca2+ + Mg2+)-ATPase activity was shown to be inversely proportional to the cholesterol concentration of the membranes, showing a low ATPase activity with a high cholesterol content and a high ATPase activity when the cholesterol concentration was low. Although the (Ca2+ + Mg2+)-ATPase activity was found to be inhibited in the cholesterol incorporated sarcolemmal sacs, the withdrawal of small amounts of cholesterol from the membranes produced an important stimulatory effect. Changes in (Ca2+ + Mg2+)-ATPase activity due to variation in the membrane cholesterol concentration were shown to be reversible. Our results indicate the possibility of a slow exchange of cholesterol between the tightly bound lipid surrounding the (Ca2+ + Mg2+)-ATPase and the bulk lipid of the sarcolemma.

Biochemical properties of membranes isolated from calcium-depleted rabbit hearts

The purpose of this study was to define the biochemical properties of sarcolemma from the calcium-depleted rabbit heart. Calcium repletion after calcium-free perfusion results in irreversible damage to the heart (calcium paradox). No difference was found in specific activity of the Na+ -Ca++ antiporter in a crude preparation of sarcolemmal vesicles that was isolated from calcium-depleted hearts, compared with control perfused hearts. Likewise, the passive calcium efflux from sarcolemmal vesicles, preloaded with calcium via the Na+ -Ca++ antiporter, showed rates that were identical with control values. This indicates that the sarcolemma calcium permeability is not affected by calcium-free perfusion of the heart. Na+,K+ -ATPase activity in sarcolemma isolated from calcium-depleted hearts was reduced by 75% (P less than 0.005) compared with the control activity. Sarcolemmal phosphoproteins, whether produced by endogenous cyclic AMP- or calcium-calmodulin-dependent protein kinase, were not altered by calcium-free perfusion of the heart. The content of an important calcium-binding site in the myocardial cell, the sialic acid residues, was also estimated. Only a long period (60 minutes) of calcium-free perfusion resulted in a significant decrease (by 68%, P less than 0.025) of sialic acid content in the homogenate but not in the sarcolemma preparation. In hearts that were reperfused for 15 minutes with a normal calcium concentration (1.3 mM), sarcolemmal Na+,K+ -ATPase remained depressed and calcium permeability was still unchanged. It is possible that the sarcolemma isolation method selected a distinct part of the sarcolemma from the calcium-depleted and repleted heart that had no modified glycocalyx and permeability barriers to calcium ions, and that another part of the sarcolemma with altered properties was lost during the isolation procedure. Another possibility is that reconstitution processes during isolation affected membrane permeability properties. The results of the Na+,K+ -ATPase measurements provide evidence that the net calcium gain of the cells after calcium repletion may be associated, in part, with a loss in ability of the sarcolemma to remove calcium from the cytosol.

The effect of anions and cations on cardiac adenylate cyclase: interactions with isoprenaline and GTP

Anions stimulated rabbit heart adenylate cyclase in the order chloride greater than phosphate greater than acetate, while the cations sodium and potassium had little effect. Activation by KCI was multiplicative with that of fluoride. Isoprenaline (10(-4) M) and GTP(10(-4) M) stimulated the cyclase independently, and their combined effects were multiplicative. GTP stimulated less than chloride but more than acetate. Activation by chloride or acetate was more than additive with that by GTP. The effect of isoprenaline was reduced in the presence of chloride or acetate, and this reduction was not modified by sodium or potassium. It was concluded that ionic influence may be biologically important in modifying cyclase activity and the extent of beta-receptor stimulation.

Mechanism of passive Ca2+ permeability of vesicular sarcolemmal preparations from rat hearts

Vesicular sarcolemmal preparations isolated from rat hearts were characterized by high total ATPase (4.32 +/- 0.57 mumol/min per mg), adenylate cyclase (121 +/- 11 pmol/min per mg) and creatine kinase (1.73 +/- 0.35 mumol/min per mg) activities as well as Na-Ca exchange specific to sodium. ATPase activity was inhibited with digitoxigenin by 50-70% and was not changed by ouabain, ionophore A23187 or oligomycin. Sarcolemmal vesicles bound [3H]digitoxigenin and [3H]ouabain in isotonic medium in the presence of Pi and Mg2+. The number of binding sites for hydrophobic digitoxigenin (N = 237 pmol/mg) was several-times higher than that for hydrophilic ouabain (N = 32.7 pmol/mg). These data show that sarcolemmal preparations were not significantly contaminated by mitochondria and sarcoplasmic reticulum and consisted mostly of inside-out vesicles. Incubation of these vesicles with 45Ca2+ (0.5-10 mM) led to penetration of the latter into the vesicles with the following binding characteristics: number of binding sites (N = 20.5 +/- 4.6 nmol/mg, Kd approximately equal to 2.0 mM). Ca2+ binding to the inner surface of vesicles was proved by the following facts: (1) Ca2+ ionophore A23187 increased slightly total intravesicular Ca2+ content but markedly accelerated Ca2+ efflux along its concentration gradient; (2) gramicidin and osmotic shock showed a similar accelerating effect. Ca2+ efflux from the vesicles along its concentration gradient ([Ca2+]i/[Ca2+]e = 2.0 mM/0.1 microM) was inhibited by Mn2+, Co2+, and verapamil when they acted inside the vesicles. The rate of Ca2+ efflux was hyperbolically dependent on intravesicular Ca2+ concentration (Km approximately equal to 2.9 mM). These data reveal that Ca2+ efflux from sarcolemmal vesicles is controlled by Ca2+ binding to the sarcolemmal membrane. Ca2+ efflux from the vesicles was stimulated 1.7--times after incubation of vesicles with 0.2 mM MgATP or MgADP and 15-times after treatment with 0.2 mM adenylyl beta, gamma-imidodiphosphate. Enhancement in the rate of Ca2+ efflux correlated with the increase in the intravesicular Ca2+ content. ATP-stimulated Ca2+ efflux was suppressed by verapamil and was nonmonotonically dependent upon the transmembrane potential created by the K+ concentration gradient in the presence of valinomycin, Ca2+ efflux being slower at extreme values of membrane potential (+/- 80 mV).

Uptake of bepridil into isolated ventricular myocytes. Retention by actin

Isolated rat ventricular myocytes accumulate large amounts of bepridil intracellularly. The mode of transport is uncertain but uptake is of such a magnitude as to mask possible sarcolemmal binding sites. Bepridil is tightly bound within myocytes, there being only a minimal efflux into bepridil-free medium. Purified F-actin binds bepridil in excess of 2 moles/mole actin, sufficient to account for the uptake of bepridil by myocytes incubated in 10 microM drug. At higher bepridil concentrations the amount transported into myocytes suggests that the drug may be distributed between actin-bound and cytoplasmic pools. Bepridil may alter cardiac contractility by a direct influence on the contractile filaments.

Regulation of the (Ca2+ + Mg2+)-ATPase activity and calcium transport in reconstituted cardiac sarcolemmal vesicles. Effect of sodium and potassium

A reconstitution procedure for a cardiac sarcolemmal enriched fraction is described. In the reconstituted cardiac sarcolemmal inside-out vesicles, a difference in calcium transport and (Ca2+ + Mg2+)-ATPase activity was found depending on the side of the membrane at which sodium and potassium were placed. Having inhibited the (Na+ + K+)- ATPase activity with ouabain, the active transport of calcium was increased when potassium was located outside and sodium inside the reconstituted vesicles. Nevertheless, this activity was maximal having potassium present on both sides. During calcium transport it was also shown that 86Rb moves opposite to calcium. When the experiment was carried out having 22Na located at the inside, there was no movement of this cation despite the low calcium transport observed. The present study supports the possibility of potassium having a stimulatory effect upon the sarcolemmal (Ca2+ + Mg2+)-ATPase activity and suggests the existence of a Ca2+, K+ co-transport carried out by this enzyme.

Partial purification of an intercalated disc-containing cardiac plasma membrane fraction

The sarcolemma of cardiac muscle cells contains a specialised junctional region, the intercalated disc which includes three types of intercellular junction, the macula and fascia adherens and the nexus or gap junction. To facilitate the isolation of these junctions a procedure for the partial purification from mouse hearts of a subcellular fraction containing the intercalated disc region of the sarcolemma was developed. This involved investigating methods of tissue disruption that preserve the integrity of the intercalated disc and minimise myofibrillar entrapment of organelles. Examination of the distribution of marker enzymes showed that relative to the homogenate the intercalated disc fraction prepared by sucrose density centrifugation was only enriched 1.5- to 3-fold in 5'-nucleotidase and (Na+ + K+)-ATPase activities, whereas mitochondrial and sarcoplasmic reticulum marker enzymes were low. The properties of the intercalated disc-containing fraction were compared with the vesicular sarcolemmal fractions devoid of junctional complexes prepared by other methods.

The effect of verapamil on the Ca2+-transporting and Ca2+-ATPase activity of isolated cardiac sarcolemmal preparations

1 The effect of (+/-)-, (+)- and (-)-verapamil on the Ca2+-binding, Ca2+-transporting activity, and Ca2+-dependent adenosine triphosphatase (ATPase) activity of isolated cardiac sarcolemmal preparations was studied. Enzymatic treatment was used to establish the nature of the sites facilitating [14C]-(+/-)-verapamil binding. 2 (+/-)-Verapamil 1 microM inhibited the passive binding of 45Ca2+. The (+/-)- and (-)-isomers were equiactive. 3 (+/-)-Verapamil 1 microM inhibited the ATP-dependent transport of 45Ca2+ and the associated activation of the Ca2+-sensitive ATPase. The activity resided in the (-)-isomer. 4 Lineweaver-Burk plots for the initial rates of ATP-dependent transport showed that the inhibition induced by the (-)-isomer was accompanied by a reduced Km and Vmax. 5 Enzymatic removal of N-acetyl neuraminic acid and galactose residues increased [14C]-(+/-)-verapamil binding; removal of N-acetylglucosamine and treatment with phospholipase C and trypsin decreased the binding. 6 These results have been interpreted to mean that (-)-verapamil interferes with the ATP-dependent Ca2+-transporting properties of the sarcolemma, and that this effect is accompanied by an altered activity of the intrinsic Ca2+-sensitive ATPase. N-acetylneuramic acid and galactose residues do not provide binding sites for verapamil at the cell surface.