Parasympathetic inhibition of sympathetic effects on pacemaker location and rate in hearts of anesthetized dogs

A cholinergic neuroskeletal interface promotes bone formation during postnatal growth and exercise

The autonomic nervous system is a master regulator of homeostatic processes and stress responses. Sympathetic noradrenergic nerve fibers decrease bone mass, but the role of cholinergic signaling in bone has remained largely unknown. Here, we describe that early postnatally, a subset of sympathetic nerve fibers undergoes an interleukin-6 (IL-6)-induced cholinergic switch upon contacting the bone. A neurotrophic dependency mediated through GDNF-family receptor-α2 (GFRα2) and its ligand, neurturin (NRTN), is established between sympathetic cholinergic fibers and bone-embedded osteocytes, which require cholinergic innervation for their survival and connectivity. Bone-lining osteoprogenitors amplify and propagate cholinergic signals in the bone marrow (BM). Moderate exercise augments trabecular bone partly through an IL-6-dependent expansion of sympathetic cholinergic nerve fibers. Consequently, loss of cholinergic skeletal innervation reduces osteocyte survival and function, causing osteopenia and impaired skeletal adaptation to moderate exercise. These results uncover a cholinergic neuro-osteocyte interface that regulates skeletogenesis and skeletal turnover through bone-anabolic effects.

"Zone of avoidance": RR interval distribution in tachograms, histograms, and Poincaré plots of a Boxer dog

The RR intervals of sinus and ventricular beats were determined by analysis of a 24-h ambulatory electrocardiogram in a Boxer before and after treatment with sotalol. These RR intervals were plotted using tachograms, histograms, and Poincaré plots. The tachogram demonstrated a 'band' wherein a range of RR intervals was infrequent, the histogram did not take the form of a single Gaussian distribution of RR intervals, and the Poincaré plot showed nonhomogeneous beat-to-beat variability. This type of patterning was described as a "zone of avoidance" potentially caused by the clustering of beats within specific ranges. Treatment with sotalol enhanced the "zone of avoidance". Further investigation is needed to understand the mechanism for this observation as well as any clinical implications.

Heterogeneous distribution of beta-adrenoceptors and muscarinic receptors in the sinoatrial node and right atrium of the dog

1. The pacemaker site is known to shift away from the sinoatrial (SA) node in response to autonomic stimuli. 2. To test whether the shifting of the impulse-initiation site that occurs during autonomic nerve stimulation can be explained by the spatial distributions of beta-adrenoceptor and muscarinic receptors, we obtained densities (B(max)) and dissociation constants (K((d)) for these receptors in three regions along the sulcus terminalis (the SA node itself and regions superior and inferior to it) and a region of the right atrial appendage in the dog. 3. The Bmax values for [(125)I]-iodocyanopindolol binding to beta-adrenoceptors were not different among the four regions. The Bmax value for [(3)H]-quinuclidinyl benzilate binding to muscarinic receptors was significantly higher in the SA node area than in any other region (P < 0.01), although there was no difference among the other three regions. 4. For each receptor, K(d) values were similar among the four regions. 5. These results suggest that spatial variations in the densities of beta-adrenoceptors and muscarinic receptors are not important for shifting the impulse-initiation site and that other factors, such as non-uniform innervation by autonomic nerve fibres, may be mainly responsible for the pacemaker shift induced by autonomic nerve stimulation.