The heart reinnervates after transplantation☆
Article InfoFig 1
Holter monitor electrocardiographic data obtained from (A, B) 2 conscious animals 1 year after cardiac acute transplantation. The upper and lower panels on the left illustrate data obtained during the slowest (A, 26 bpm; B, 31 bpm) and fastest (A, 163 bpm; B, 120 bpm) heart rates in each dog. To the right are power spectral data (PSD) derived from representative 20-minute periods when bradycardia occurred in both animals. These data display similar power spectral peaks despite the fact that the heart of the dog in A exhibited no evidence of parasympathetic reinnervation, whereas that of the dog in B was reinnervated by parasympathetic efferent neurons.
Fig 2
Varied cardiac effects elicited in the 2 dogs in Figure 1 (A and B represent the same animals) induced by stimulating efferent axons in sectioned right (RV) or left (LV) cervical vagus individually. The times when stimuli were applied to neural tissues are indicated by the solid black sections (stimulus artifacts) recorded in the electrocardiographic traces. Note that bradycardia was elicited in 1 animal (B) but not in the other (A). (ECG = lead II electrocardiogram; LVP = left ventricular chamber pressure.)
Fig 3
Gross and microscopic analyses of suture lines 1 year after transplantation. (A) Gross anatomy of suture line on intimal surface of ascending aorta. (B) Masson’s trichrome stain of suture line of aorta (Ao) and pulmonary artery (PA) obtained from tissue specimen in A. (C) Neurofilament staining of same tissue demonstrating a relatively large nerve coursing over suture line to cardiac tissue at bottom of Figure. (D) Enlargement of tissue in boxed area of C. Note the deficit where a suture was located (small arrow) and axons coursing through the suture line (large arrow). Calibration bars: B and C = 10 mm; D = 2 mm.
Fig 4
Cardiac effects elicited by stimulating decentralized (A) left or (B) right stellate ganglion individually in animal with substantial reinnervation. Heart rate increased during right stellate ganglion stimulation, and left ventricular systolic pressure rose when either stellate ganglion was stimulated. (ECG = lead II electrocardiogram; LV IMP = left ventricular intramyocardial pressure; LVP = left ventricular chamber pressure.)
Fig 5
Responses displayed by two inferior vena cava–inferior right atrial neurons to intracellular or connecting nerve stimulation in vitro. (A) An action potential (upper trace) was generated by a depolarizing current pulse (1-ms duration) (lower trace) delivered into a neuron by way of the intracellular recording electrode. (B) A synaptically generated action potential occurred in the same neuron after stimulating a nerve (arrow stim) connected to the ganglion containing the identified neuron (arrow) before (Pre Hex) but not after (Hex) hexamethonium administration. Vertical bars: A = 10 mV; B = 20 mV; horizontal bars: A and B = 10 ms.
Fig 6
Saturable specific binding of [3H]CGP12177 to tissues (2 mm in diameter, 350 μm thick) obtained from left ventricles of control (○) and transplanted (•) hearts. Combined data derived from the ventricles of 4 study dogs and 3 control dogs are shown.
Fig 7
(A) Northern blot analysis was performed using specific complementary DNA probes; 18S ribosomal RNA was used as an internal control. Expression levels of messenger RNA for both α-tubulin (α-Tub) and β-tubulin (β-Tub) were augmented in myocardial tissues obtained from two transplanted hearts (T4, T6) compared with levels in control heart tissue (C). Myocardial tissue mRNA was substantially degraded in tissues obtained from a third animal (ie, T3; blot not shown). (B) Immunoblot analysis of tissue from the same myocardium demonstrating either a substantial decrease (ie, T3, T4) or no change (ie, T6) in total tubulin levels compared with control myocardial tissue. Myosin heavy chain (MHC) from a corresponding Coomassie blue–stained gel is also shown.
Abstract
Background. Whether cardiac reinnervation occurs after transplantation remains controversial. If reinnervation does occur, how sympathetic and parasympathetic efferent neurons do this remains unknown.
Methods. Power spectral analysis of heart rate variability was assessed for 1 year after cardiac autotransplantation in 9 dogs. After induction of anesthesia 13 months after transplantation, cardiac and intrinsic cardiac neuronal responses elicited by both electrical stimulation of parasympathetic or sympathetic efferent neurons and systemic or local coronary artery administration of nicotine (5 μg/kg), angiotensin II (0.75 μg/kg), and tyramine (1.2 μg/kg) were studied. The transmembrane electrical properties of intrinsic cardiac neurons were studied in vitro. Ventricular tissue catecholamine content, α-tubulin expression, and β-adrenergic receptor density and affinity were studied. The presence of axons crossing suture lines was sought histologically.
Results. Nerves were identified crossing suture lines. Electrical or chemical (ie, nicotine or angiotensin II) activation of sympathetic efferent neurons enhanced cardiodynamics, as did tyramine. Stimulating vagal efferent preganglionic axons induced bradycardia in half of the dogs. Functional reinnervation did not correlate with specific power spectra derived from rate variability in the conscious state. Responding to nicotine and angiotensin II in situ, transplanted intrinsic cardiac neurons generated spontaneous activity. These neurons displayed nicotine-dependent synaptic inputs in vitro. Ventricular tissue had normal β-adrenergic receptor affinity and density but reduced catecholamine and α-tubulin contents.
Conclusions. The intrinsic cardiac nervous system receives reduced input from extracardiac sympathetic efferent neurons after transplantation and inconsistent input from parasympathetic efferent preganglionic neurons. These heterogeneous neuronal inputs are not reflected in heart rate variability or ventricular β-adrenergic receptor function. Transplanted angiotensin II–sensitive intrinsic cardiac neurons exert greater cardiac control than do nicotine-sensitive ones. The intrinsic cardiac nervous system remodels itself after cardiac transplantation, and this indicates that direct assessment of extracardiac and intrinsic cardiac neuronal behavior is required to fully understand cardiac control after transplantation.
☆This article has been selected for the open discussion forum on the STS Web site: http://www.sts.org/section/atsdiscussion/
