ABSTRACT
Whole-cell recording methods and fluorescence microscopy were used to study the effects of acute exposure to thyroid hormone (T(3)) on cat atrial myocytes. Acute exposure ( approximately 5 min) to 10 nM T(3) significantly increased tetrodotoxin (TTX)-sensitive inward Na(+) current (I(Na)) at voltages between -40 and +20 mV. At maximal I(Na) activation (-40 mV) T(3) increased peak I(Na) by 32 %. T(3) had no effect on the time course of I(Na) decay, voltage dependence of activation, inactivation, or recovery from inactivation. Comparable exposures to reverse T(3) (rT(3)) or T(4) had no effect on I(Na). L-type Ca2+ current was unaffected by acute exposure to T(3). T(3)-induced increases in I(Na) were unaffected by 50 microM nickel, a blocker of T-type Ca2+ current. T(3) significantly increased cell shortening (+62 %) and could elicit spontaneous action potentials arising from Ca2+ -mediated after-depolarizations. T(3) (but not rT(3)) significantly increased baseline intracellular Ca2+, release of Ca2+ from sarcoplasmic reticulum (SR) and caffeine (10 mM)-induced release of SR Ca2+. We conclude that acute T(3) exposure increases Na(+) influx via I(Na) and thereby stimulates reverse-mode Na(+)-Ca2+ exchange to increase intracellular Ca2+ content and release. As a result, T(3) increases contraction strength, and can initiate Ca2+ -mediated arrhythmic activity. Acute non-genomic effects of T(3) can contribute to the positive inotropy and sinus (atrial) tachycardia traditionally attributed to chronic, genomic effects of elevated thyroid hormone on atrial muscle.
Subject(s)
Calcium/metabolism , Intracellular Membranes/metabolism , Myocytes, Cardiac/metabolism , Sodium Channels/physiology , Triiodothyronine/pharmacology , Action Potentials/drug effects , Animals , Caffeine/pharmacology , Cats , Electric Conductivity , Female , Heart Atria , In Vitro Techniques , Male , Myocardial Contraction/drug effects , Myocytes, Cardiac/drug effects , Sarcoplasmic Reticulum/drug effects , Sarcoplasmic Reticulum/metabolism , Sodium Channels/drug effects , Tetrodotoxin/pharmacologyABSTRACT
In atrial myocytes, an initial exposure to isoproterenol (ISO) acts via cAMP to mediate a subsequent acetylcholine (ACh)-induced activation of ATP-sensitive K(+) current (I(K,ATP)). In addition, beta-adrenergic receptor (beta-AR) stimulation activates nitric oxide (NO) release. The present study determined whether the conditioning effect of beta-AR stimulation acts via beta(1)- and/or beta(2)-ARs and whether it is mediated via NO signaling. 0.1 microM ISO plus ICI 118,551 (ISO-beta(1)-AR stimulation) or ISO plus atenolol (ISO-beta(2)-AR stimulation) both increased L-type Ca(2+) current (I(Ca,L)) markedly, but only ISO-beta(2)-AR stimulation mediated ACh-induced activation of I(K,ATP). 1 microM zinterol (beta(2)-AR agonist) also increased I(Ca,L) and mediated ACh-activated I(K,ATP). Inhibition of NO synthase (10 microM L-NIO), guanylate cyclase (10 microM ODQ), or cAMP-PKA (50 microM Rp-cAMPs) attenuated zinterol-induced stimulation of I(Ca,L) and abolished ACh-activated I(K,ATP). Spermine-NO (100 microM; an NO donor) mimicked beta(2)-AR stimulation, and its effects were abolished by Rp-cAMPs. Intracellular dialysis of 20 microM protein kinase inhibitory peptide (PKI) abolished zinterol-induced stimulation of I(Ca,L). Measurements of intracellular NO ([NO](i)) using the fluorescent indicator DAF-2 showed that ISO-beta(2)-AR stimulation or zinterol increased [NO](i). L-NIO (10 microM) blocked ISO- and zinterol-induced increases in [NO](i). ISO-beta(1)-AR stimulation failed to increase [NO](i). Inhibition of G(i)-protein by pertussis toxin significantly inhibited zinterol-mediated increases in [NO](i). Wortmannin (0.2 microM) or LY294002 (10 microM), inhibitors of phosphatidylinositol 3'-kinase (PI-3K), abolished the effects of zinterol to both mediate ACh-activated I(K,ATP) and stimulate [NO](i). We conclude that both beta(1)- and beta(2)-ARs stimulate cAMP. beta(2)-ARs act via two signaling pathways to stimulate cAMP, one of which is mediated via G(i)-protein and PI-3K coupled to NO-cGMP signaling. Only beta(2)-ARs acting exclusively via NO signaling mediate ACh-induced activation of I(K,ATP). NO signaling also contributes to beta(2)-AR stimulation of I(Ca,L). The differential effects of beta(1)- and beta(2)-ARs can be explained by the coupling of these two beta-ARs to different effector signaling pathways.
