Achilles J. Pappano

Calcium-Dependent Action Potentials Produced by Catecholamines in Guinea Pig Atrial Muscle Fibers Depolarized by Potassium

Atrial muscle fibers of the guinea pig were depolarized and rendered inexcitable by elevation of [K+]o to 22 mM. Isoproterenol, epinephrine, and norepinephrine restored propagated action potentials and contractions to atrial cells without changing the resting potential. The peak value of the intracellularly recorded action potentials varied 28 mv for a 10-fold change in [Ca2+]o. Isoproterenol-induced restoration of action potentials was insensitive to large concentrations of tetrodotoxin (3 × 10−6M) but antagonized by Mn2+ (0.2 mM). These data supported the proposal that the catecholamines restored excitability by increasing membrane conductance to Ca2+. In addition to the catecholamines, the divalent cations Ba2+, Sr2+, and Ca2+ restored action potentials to atrial muscle fibers depolarized by elevated K+. The effectiveness of the divalent ions in allowing action potentials was inversely related to the estimated hydrated ionic radii. Like the action potentials observed in the presence of isoproterenol, those permitted by Ba2+, Sr2+, and Ca2+ were prevented by Mn2+ but insensitive to blockade by tetrodotoxin.

Cellular Actions and Pharmacology of the Calcium Channel Blocking Drugs

The calcium channel blockers represent a group of diverse chemical structures that block calcium-selective channels in the plasma membranes of a variety of excitable cells. As the calcium fluxes carried by these channels allow the calcium ion (Ca2+) to gain access to the cell interior, where calcium serves as an activator messenger, calcium channel blockers generally act to inhibit cell function. By reducing the depolarizing currents caused by the entry of positively charged Ca2+ into the negatively charged interior of resting cells, the calcium channel blockers also inhibit excitatory processes that depend on calcium entry across the plasma membrane. These principles account for most of the effects of calcium channel blockers on the cardiovascular system. The calcium channel blockers inhibit contractile function in the heart and vascular smooth muscle and, because the initial depolarizing currents in the sinoatrial and atrioventricular nodes are carried by calcium channels, slow the heart rate and prolong atrioventricular conduction. The negative inotropic and vasodilatory effects of the calcium channel blockers, both of which can reduce systemic blood pressure, offer a theoretic basis for their potential use in the treatment of hypertension. The tissue specificity exhibited by some of the calcium channel blockers may enhance their therapeutic value in selected hypertensive patients. Of the three calcium channel blockers now available for use in the United States (diltiazem, nifedipine, and verapamil), diltiazem and verapamil are approximately equipotent in inhibiting calcium channel function in the heart and vascular smooth muscle, whereas nifedipine is more potent in smooth muscle. This tissue specificity can be used to advantage in the management of hypertension. These pharmacologic principles underlie the growing appreciation of the potential value of the calcium channel blockers in the treatment of hypertension.

Pertussis toxin-insensitive phosphoinositide hydrolysis, membrane depolarization, and positive inotropic effect of carbachol in chick atria.

Muscarinic agonists can stimulate rather than inhibit cardiac muscle in some preparations. In left atria from hatched chicks, treatment with pertussis toxin reversed the membrane action of carbachol from hyperpolarization to depolarization and reversed the inotropic effect of carbachol from negative to positive. Acetylcholine also depolarized the membrane and increased the force of contraction in atria from pertussis-toxin-treated chicks although oxotremorine did not. These cholinergic responses were blocked by atropine but not by adrenoceptor antagonists, suggesting that they are mediated via muscarinic receptors and are not due to actions of endogenously released catecholamines. Muscarinic receptor stimulation leads to two distinct biochemical responses in chick atria: inhibition of adenylate cyclase and activation of phosphoinositide (PI) hydrolysis. The former is lost in atria from pertussis-toxin-treated chicks, whereas the PI response persists. The pharmacologic characteristics of the PI response resemble those of the depolarization and positive inotropic response. Both are insensitive to blockade by pertussis toxin, require high concentrations of carbachol, and are elicited by acetylcholine but not by oxotremorine. The present study suggests that muscarinic agonist-induced PI turnover may be responsible for the membrane depolarization and positive inotropic effects of carbachol and acetylcholine; that an increase in Na+ conductance underlies these responses; and that it is stimulated either by an increase of intracellular calcium mobilized by inositol triphosphate and/or by activation by protein kinase C.

Inositol trisphosphate promotes Na-Ca exchange current by releasing calcium from sarcoplasmic reticulum in cardiac myocytes.

An early inward tail current evoked by membrane depolarization (from -80 to -40 mV) sufficient to activate sodium but not calcium current was studied in single voltage-clamped ventricular myocytes isolated from guinea pig hearts. Like forward-mode Na-Ca exchange, this early inward tail current required [Na+]o and [Ca2+]i and is thought to follow earlier reverse-mode Na-Ca exchange that triggers Ca2+ release from sarcoplasmic reticulum. The dependence of the early inward tail current on [Ca2+]i was supported by the ability of small (+10 mV) and large (+80 mV) voltage jumps from -40 mV to decrease and increase, respectively, the size of early inward tail currents evoked by subsequent voltage steps from -80 to -40 mV. As expected, tetrodotoxin selectively inhibited the early inward tail current but not the late inward tail current that followed voltage jumps to +40 mV test potentials. Although tetrodotoxin also blocked the fast Na+ current, replacement of extracellular Na+ by Li+ sustained the fast Na+ current. However, Li+, which does not support Na-Ca exchange, reversibly suppressed both the early and late inward tail currents. Inhibitors (ryanodine and caffeine) and promoters (intracellularly dialyzed inositol 1,4,5-trisphosphate) of sarcoplasmic reticulum Ca2+ release decreased and increased, respectively, the magnitude of the early inward tail current. The results substantiate the hypothesis that Ca2+ release from the sarcoplasmic reticulum participates in early Na-Ca exchange current and demonstrate that inositol 1,4,5-trisphosphate, by releasing Ca2+ from the sarcoplasmic reticulum, can promote Na-Ca exchange across the plasma membrane.

Extracellular ATP-stimulated current in wild-type and P2X4 receptor transgenic mouse ventricular myocytes: implications for a cardiac physiologic role of P2X4 receptors

P2X receptors, activated by extracellular ATP, may be important in regulating cardiac function. The objective of the present study was to characterize the electrophysiologic actions of P2X4 receptors in cardiac myocytes and to determine whether they are involved in mediating the effect of extracellular ATP. Membrane currents under voltage clamp were determined in myocytes from both wild‐type (WT) and P2X4 receptor‐overexpressing transgenic (TG) mice. The P2X agonist 2‐meSATP induced an inward current at −100 mV that was greater in magnitude (2‐fold) in TG than in WT ventricular cells. In the presence of the P2X4 receptor‐selective allosteric enhancer ivermectin (3 μM), the 2‐meSATP‐stimulated current increased significantly in both WT and TG ventricular cells, consistent with an important role of P2X4 receptors in mediating the ATP current not only in TG but also WT myocytes. That the current in both WT and TG cells showed similar voltage‐dependence and reverse potential (∼0 mV) further suggests a role for this receptor in the normal electrophysiological action of ATP in WT murine cardiac myocytes. The P2X antagonist suramin was only able to block partially the 2‐meSATP‐stimulated current in WT cells, implying that both P2X4 receptor and another yet‐to‐be‐identified P2X receptor mediate this current.—Shen, J.‐B., Pappano, A. J., Liang, B. T. Extracellular ATP‐stimulated current in wild‐type and P2X4 receptor transgenic mouse ventricular myocytes: implications for a cardiac physiologic role of P2X4 receptors. FASEB J. 20, 277–284 (2006)

Epinephrine and the pacemaking mechanism at plateau potentials in sheep cardiac Purkinje fibers.

Abstract1.In 1.35 mM [K+]o, sheep cardiac Purkinje fibers depolarized to about −40 mV. Whereas some fibers oscillated spontaneously at plateau potentials, others could be made to oscillate when polarized by intracellular currents. Pacemaker activity at plateau potentials (−50 to 0 mV) was distinct from that caused by the

Role of P2X purinergic receptors in the rescue of ischemic heart failure

i_{{\text{K}}_2 }

Vagal Stimulation of the Heartbeat: Muscarinic Receptor Hypothesis

pacemaker at more negative potentials (−60 to −100 mV).2.Epinephrine induced spontaneously occurring action potentials and increased pacemaker activity in depolarized Purkinje fibers. The ED50 for the positive chronotropic effect of epinephrine was about 5×10−7 M. This concentration is similar to that reported for the effect of epinephrine on plateau amplitude (Carmeliet and Vereecke, 1969) and the slow inward current (isi, Reuter, 1974).3.In voltage clamp experiments, epinephrine increased the magnitude ofisi and of an outward plateau current,ixi. It is concluded that epinephrine effects pacemaking at plateau potentials by increasingisi and without shifting the voltage dependence of these currents. The onset of pacemaker activity by epinephrine was preceded by membrane depolarization that results from an inward shift of the steady-state current-voltage relation. This current may flow throughisi channels that are activated but not completely inactivated.