Adrenoceptor sub-type involvement in Ca2+ current stimulation by noradrenaline in human and rabbit atrial myocytes

Fig. 1. Noradrenaline (NA) increases human atrial L-type Ca2+ current (ICaL) in a concentration-dependent manner. A Superimposed original representative peak ICaL traces, recorded from a single human atrial myocyte by stepping voltage to 0 mV (from HP − 50 mV) following acute superfusion of NA at concentrations shown. B NA concentration-ICaL density relationship. Values are means ± SE; n = 8–31cells, 4–8 patients. Curve-fit: sigmoidal; variable slope, 4 parameters, no constraints, accounting for n and scatter amongst replicates. Emax and Emin: ICaL at maximally and minimally effective [NA]; EC50 and EC75: 50% and 75% maximally-effective [NA], respectively

Fig. 2. Effects of broad action β- and α-adrenoceptor (AR) antagonists on NA-stimulated ICaL in human and rabbit atrial myocytes. A Typical time course of changes in peak ICaL recorded from two human atrial myocytes (i and ii), before (control: “Con”) and during step-wise cumulative addition of noradrenaline (310 nM; EC75: “NA”), propranolol (0.2 µM; β1 + β2-AR antagonist: “Pro”) and phentolamine (1 µM; α1 + α2-AR antagonist: “Phe”). Vertical dashed lines: start time of NA or antagonist addition to (or washout from) perfusion bath. (i and ii) NA-stimulated ICaL is decreased by Pro, then further by Phe; ii shows partial recovery of NA effect after drug washout. B Corresponding average (mean ± SE; with individual points shown) magnitude of responses to NA and AR antagonists as used in A. * = P < 0.05 (ANOVA); n = 9 cells, from 3 patients. C Corresponding rabbit atrial ICaL time courses, in two myocytes (i and ii), with Phe having opposite effects (both reversible) between them. D Average effects of NA and AR antagonists as used in C, in 7 cells, from 4 rabbits. NS = not significant

Fig. 3. Comparison of independent anti-adrenergic effects of propranolol and phentolamine. A Representative time courses of ICaL change, in two rabbit atrial myocytes (i and ii), after adding 310 nM NA then either (i) 0.2 µM Pro or (ii) 1 µM Phe. B Corresponding mean ± SE (with individual points shown) responses in (i) Pro group (n = 6 cells, 3 rabbits) and (ii) Phe group (9 cells, 2 rabbits). * = P < 0.05: ANOVA within Pro and Phe groups; un-paired t-test between them (NS = not significant)

Fig. 4. Comparison of β-AR sub-type contributions to ICaL-stimulation by NA, between human and rabbit atrial myocytes. A ICaL time course changes in two human atrial cells (i and ii) in response to a β1-antagonist (CGP20712A at 0.3 µM: “CGP”), then a β2-antagonist (ICI118551 at 0.1 μM: “ICI”), both in the presence of 310 nM NA. (i and ii) NA-stimulated ICaL is decreased by CGP, then either reversibly increased (i) or decreased (ii) by ICI. B Mean effects of interventions in A. n = 5–6 cells, 2 patients; * = P < 0.05, NS = not significant (ANOVA). C Corresponding ICaL time course changes in two representative rabbit atrial cells: in i and ii, CGP again decreased ICaL, but ICI consistently increased it, confirmed by D mean data (showing individual points; n = 5 cells, 4 rabbits)

Fig. 5. α-AR sub-type contributions to NA-stimulation of ICaL in human and rabbit atrial myocytes. A Typical ICaL changes in two human atrial cells (i and ii) in response to an α1-antagonist (prazosin, 0.5 µM: “Pra”), then an α2-antagonist (yohimbine, 10 µM: “Yoh”), both with NA at 310 nM. B Mean responses to interventions in A. n = 6 cells, 3 patients; * = P < 0.05, NS = not significant (ANOVA). C Corresponding ICaL changes in two rabbit atrial cells (i and ii). D Mean responses (n = 7 cells, 3 rabbits) to same interventions as in C.