title: Rad GTPase deletion increases L-type calcium channel current leading to increased cardiac contraction.
abstract: BACKGROUND
  The small GTPase Rad is a negative regulator of voltage-dependent L-type calcium channel current (ICaL); however, the effects of Rad ablation on cardiomyocyte function are unknown. The objective of this study is to test the hypothesis that Rad-depletion causes positive inotropic effects without inducing cardiac hypertrophy.
  
  
  RESULTS
  Ventricular myocytes from adult Rad(-/-) mice were isolated and evaluated by patch-clamp recordings for I(Ca,L) and action potentials, Ca(2+) transients, and sarcomere shortening. Maximum I(CaL) is elevated in Rad(-/-) (maximal conductance 0.35 ± 0.04 picoSiemens/picoFarad (pS/pF) wild-type; 0.61 ± 0.14 pS/pF Rad(-/-)), decay kinetics are faster, and I(Ca,L) activates at lower voltages (activation midpoint -7.2 ± 0.6 wild-type; -11.7 ± 0.9 Rad(-/-)) mimicking effects of β-adrenergic receptor stimulation. Diastolic and twitch calcium are elevated in Rad(-/-) (F340/380: 1.03 diastolic and 0.35 twitch for wild-type; 1.47 diastolic and 0.736 twitch for Rad(-/-)) and sarcomere shortening is enhanced (4.31% wild-type; 14.13% Rad(-/-)) at lower pacing frequencies. Consequentially, frequency-dependence of Ca(2+) transients is less in Rad(-/-), and the frequency dependence of relaxation is also blunted. In isolated working hearts, similar results were obtained; chiefly, +dP/dt was elevated at baseline and developed pressure was relatively nonresponsive to acute β-adrenergic receptor stimulation. In single cells, at subphysiological frequencies, nonstimulated calmodulin-dependent protein kinase-sensitive calcium release is observed. Remarkably, Rad(-/-) hearts did not show hypertrophic growth despite elevated levels of diastolic calcium.
  
  
  CONCLUSIONS
  This study demonstrates that the depletion of Rad GTPase is equivalent to sympathomimetic β-adrenergic receptor, without stimulating cardiac hypertrophy. Thus, targeting Rad GTPase is a novel potential therapeutic target for Ca(2+)-homeostasis-driven positive inotropic support of the heart.
authors: Department of Physiology, University of Kentucky College of Medicine, Lexington, KY.
authors: Janet R
authors: JR
authors: Manning
authors: Guo
authors: G
authors: Yin
authors: Catherine N
authors: CN
authors: Kaminski
authors: Janos
authors: J
authors: Magyar
authors: Han-Zhong
authors: HZ
authors: Feng
authors: John
authors: J
authors: Penn
authors: Gail
authors: G
authors: Sievert
authors: Katherine
authors: K
authors: Thompson
authors: J-P
authors: JP
authors: Jin
authors: Douglas A
authors: DA
authors: Andres
authors: Jonathan
authors: J
authors: Satin
mesh_terms: D000200:Action Potentials; D000318:Adrenergic beta-Agonists / Q000494:pharmacology; D000818:Animals; D020746:Calcium Channels, L-Type / Q000378:metabolism*; D017871:Calcium-Calmodulin-Dependent Protein Kinases / Q000378:metabolism; D002304:Cardiac Pacing, Artificial; D002316:Cardiotonic Agents / Q000494:pharmacology; D056966:Excitation Contraction Coupling / Q000187:drug effects; D017353:Gene Deletion; D005838:Genotype; D006339:Heart Rate; D007700:Kinetics; D051379:Mice; D057507:Mice, 129 Strain; D008810:Mice, Inbred C57BL; D018345:Mice, Knockout; D009200:Myocardial Contraction / Q000187:drug effects; D032383:Myocytes, Cardiac / Q000187:drug effects / Q000201:enzymology*; D010641:Phenotype; D012518:Sarcomeres / Q000378:metabolism; D015854:Up-Regulation; D017725:Ventricular Pressure; D018631:ras Proteins / Q000172:deficiency* / Q000235:genetics
chemical_list: D000318:Adrenergic beta-Agonists; D020746:Calcium Channels, L-Type; D002316:Cardiotonic Agents; C490030:Rrad protein, mouse; D017871:Calcium-Calmodulin-Dependent Protein Kinases; D018631:ras Proteins
keywords: L‐type calcium current; RGK; Rad; inotropy; β‐adrenergic stimulation