Felodipine protects human atrial muscle from hypoxia-reoxygenation dysfunction: A force-frequency relationship study in an in vitro model of stunning

Aims: We aimed at investigating contractile changes after hypoxia- reoxygenation and dobutamine challenge in superfused human atrial pectinate muscle to see whether high versus low stimulation rate during hypoxia might account for outcome differences compatible with the definition of an in vitro model of myocardial stunning and whether pretreatment with the dihydropyridine Ca²⁺ entry blocker felodipine might afford protection. Methods: Human right atrial trabeculae obtained from adult patients were superfused in an organ bath with oxygenated (O₂ content 16 ml/l) and modified (NaHCO₃ 25.7 mmol/l) Tyrode's solution at 37°(2. Dobutamine (1 nmol/l to 10 μmol/l) was superfused in 10 oxygenated preparations to select the optimal drug concentration to be used in another 22 which were randomized. Group (A) consisted of time-related controls (Tyrodes's solution for 225 min at cycle length (CL) 1600 ms and no dobutamine). There were two test groups, respectively. (B) low (1600 ms CL) and (C) high (400 ms CL) stimulation rate. After 60 min of stabilization, in groups B and C, hypoxic superfusion (O₂ content 5 ml/l) lasted 60 min, then reoxygenation (60 min) and dobutamine challenge (1 μmol/l, 15 min) were performed. Analysis of variance for repeated measures with the Greenhouse-Geisser correction, and a repeated measures model with structured covariance (preparation mass, length, width and time-varying time to peak tension) matrices were used whereby grouping (G), time (T) and G T interaction were weighted. Force-frequency relationship and post-pausal potentiation were studied after each phase. Electrophysiology, histomorphometry and electron microscopy were carried out (n = 6). Felodipine (0.1 μmol/l, n = 5) pretreatment (15 min before hypoxia) was given in parallel experiments. Results: Time-related controls showed ≃10% per hour decrease of developed tension and the Paradise test provided ≃80% of control values. In test groups (as compared to baseline values) contractility was decreased ≃65% after hypoxia-reoxygenation and it increased ≃25% after dobutamine (G, 0.00652+-related diastolic processes thus helping index myocardial contractile reserve. Force-frequency relationship was negative at high stimulation rates, concomitant to an abrupt change of shape and duration of action potential with little time for Ca²⁺- related Ca²⁺ release and ensuing systolic processes. Felodipine pretreatment enabled an unblunted response to dobutamine. Histomorphometry showed an unexpected 'fibrotic core'. At electron microscopy, subendocardial and deep part of the same pectinate muscles showed identical degrees of degenerative lesions. Superfused samples showed, unexpectedly, less anoxic lesions than preparations fixed within 15 min from surgical explant, although lesions were higher than in samples fixed immediately after explant. Conclusions: This might be a relevant model, whereby pharmacological or physical interventions are tested. Native human atrial trabeculae might be used without dissection and/or preservatives. If high stimulation rate during hypoxia is used the power of hypothesis testing is maximized. Future studies with this material will be easier and comparatively smaller series might be investigated. Felodipine at low concentration was protective in this model.