Journal of Clinical Medicine Article Rho Kinase Activity, Connexin 40, and Atrial Fibrillation: Mechanistic Insights from End-Stage Renal Disease on Dialysis Patients Lorenzo A. Calò 1, *, Verdiana Ravarotto 1 , Giovanni Bertoldi 1 , Elisa Pagnin 1 , Barbara Rossi 1 , Matteo Rigato 1 , Paul A. Davis 2 and Riccardo Proietti 3 1 Department of Medicine, Nephrology, Dialysis and Transplantation Unit, University of Padova, 35128 Padova, Italy; verdiana.ravarotto@gmail.com (V.R.); giovanni.bertoldi92@gmail.com (G.B.); elisa.pagnin@unipd.it (E.P.); barbara.rossi@aopd.veneto.it (B.R.); matteo.rigato@hotmail.it (M.R.) 2 Department of Nutrition, University of California, Davis, CA 95616, USA; padavis@ucdavis.edu 3 Department of Cardiac, Thoracic, Vascular Sciences and Public Health, University of Padova, 35128 Padova, Italy; riccardoproietti6@gmail.com * Correspondence: renzcalo@unipd.it Received: 23 December 2019; Accepted: 7 January 2020; Published: 7 January 2020   Abstract: Evidence on cellular/molecular mechanisms leading to atrial fibrillation (AF) are scanty. Increased expression of Rho kinase (ROCK) and myosin-phosphatase-target subunit-1 (MYPT-1), ROCK activity’s marker, were shown in AF patients, which correlated with connexin 40 (Cx40) expression, membrane protein of heart gap junctions, key for rapid action potential’s cell–cell transfer. AF is the most frequent arrhythmia in dialysis patients who present increased MYPT-1 phosphorylation, which correlates with left ventricular (LV) mass. Given ROCK’s established role in cardiovascular–renal remodeling, induction of impaired cell-to-cell coupling/potential conduction promoting AF initiation/perpetuation, we evaluated in dialysis patients with AF, MYPT-1 phosphorylation, Cx40 expression, and their relationships to support their involvement in AF. Mononuclear cells’ MYPT-1 phosphorylation, Cx40 expression, and the ROCK inhibitor fasudil’s effect were assessed in dialysis patients with AF (DPAFs), dialysis patients with sinus rhythm (DPs), and healthy subjects (C) (western blot). M-mode echocardiography assessed LV mass and left atrial systolic volume. DPAF’s phospho-MYPT-1 was increased vs. that of DPs and C (1.57 ± 0.17 d.u. vs. 0.69 ± 0.04 vs. 0.51 ± 0.05 respectively, p < 0.0001). DP’s phospho-MYPT-1 was higher vs. that of C, p = 0.009. DPAF’s Cx40 was higher vs. that of DPs and C (1.23 ± 0.12 vs. 0.74 ± 0.03 vs. 0.69 ± 0.03, p < 0.0001). DPAF’s phospho-MYPT-1 correlated with Cx40 (p < 0.001), left atrial systolic volume (p = 0.013), and LV mass (p = 0.014). In DPAFs, fasudil reduced MYPT-1 phosphorylation (p < 0.01) and Cx40 expression (p = 0.03). These data point toward ROCK and Cx40’s role in the mechanism(s) leading to AF in dialysis patients. Exploration of the ROCK pathway in AF could contribute to AF generation’s mechanistic explanations and likely identify potential pharmacologic targets for translation into treatment. Keywords: atrial fibrillation; Rho kinase; connexin; cardiovascular–renal remodeling; renal failure 1. Introduction Atrial fibrillation (AF) is a clinically common sustained arrhythmia [1], where atrial myocytes structural and electrical remodeling and cardiac remodeling contribute greatly to its pathogenesis [2]. The renin–angiotensin–aldosterone system (RAAS) has been implicated as a major signal-processing pathway in mediating the molecular mechanisms involved in the development of atrial fibrosis and AF [3]. The identification of a central role played by RAAS in the cellular events leading to AF derives not only J. Clin. Med. 2020, 9, 165; doi:10.3390/jcm9010165 www.mdpi.com/journal/jcm