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Adrenomedullin Stimulates Porcine Sinus Node Automaticity

Stenberg TA, Koren S, Kildal AB and Myrmel T*

Adrenomedullin (AM) is a vasodilatory and chronotropic peptide that has cardioprotective properties in settings of ischemia-reperfusion. Specifically, the 52-amino acid peptide hormone has been shown to reduce infarct size and potentially be protective against malignant arrhythmias. AM reduces L-type calcium currents in rabbit ventricular myocytes, and can possibly be utilized as an antiarrhythmic drug. However, at present it is unclear whether AM has cardiac electrophysiological effects in vivo.

In the present study, we investigated whether AM affects sinus and atrioventricular nodal functions, cardiac pacing thresholds, refractory properties and intracardiac conduction intervals in an intact porcine model (Norwegian landrace and Yorkshire hybrids; n=12). Hemodynamic and electrophysiological parameters were recorded at baseline and following 60 min of AM-infusion (100 ng/kg/min).

The hemodynamic effects of AM were characterized by a reduced mean arterial pressure (76 ± 9 vs. 57 ± 6; p<0.05; mmHg), tachycardia (91 ± 13 vs. 112 ± 15; p<0.05; beats/min) and an augmented cardiac index (131 ± 20 vs. 176 ± 28; p<0.05; ml/min/kg). The sinus cycle length was reduced (674 ± 89 vs. 546 ± 73; p<0.05; ms), sinoatrial conduction time was unaltered (79 ± 24 vs. 72 ± 19; ns; ms), and the sinus node recovery time (SNRT) was reduced at paced cycle lengths of 425 ms, 375 ms and 325 ms (average SNRT 1034 ± 449 vs.704 ± 141; p<0.05; ms). Diastolic pacing thresholds, effective refractory periods, Wenckebach cycle length and intracardiac conduction intervals were unaffected by AM.

In conclusion, AM stimulates sinus node function with a reduced sinus cycle length and increased automaticity but has no further detectable intracardiac electrophysiological effects. Whether the increased automaticity is a direct effect on the sinus node or an indirect effect mediated through the autonomic nervous system remains to be determined.