Intrinsic nervous system: Differential control of cardiac contractility and automaticity

Autonomie decentralization associated with cardiac transplantation alters the neural control of the heart. It is hypothesized that remaining intrinsic cardiac nervous system (ICNS) retains a capacity for complex integrative organ regulation. In isolated rabbit hearts (n = 7) regulatory changes in contractility (intramyocardial pressure; baseline IMP=63.3± 1.6 mrnHg) and heart rate (baseline HR = 170.3± 1.9 bpm) were studied. Veratridine (V; 32 nmol) and bradykinin (B; 80 nmol) were given to activate the afferent neural fibers. Hexamethonium (HEX; 10^-4 M) was used to dissect reflex specific activity of intrinsic neural circuits. Both V and B caused positive inotropic (22.1±3.5 and 5.8± 1.2 mmHg) and negative chronotropic effects (-6.2+0.8 and -8.1± 1.0 bpm, respectively). HEX selectively diminished the effect of V on IMP (AV=23.6±6.1%; p<0.05) and the effect B on HR (AB=47.1±9.1%; p<0.05) without modifying V-chrono- and B-inotropic action. The HEX-dependent effects of V and B were uncorrelated (p > 0.05). Thus, the stimulation of specific afferent pathways activates unique neural circuits in the ICNS, enabling selective regulation of cardiac contractility and automaticity. The observed differential reflex response to different afferent stimuli (V and B) suggests that the functional organization of the ICNS involves complex interactions of intrinsic reflex loops regulating the basic properties of the heart.