Evidence for an indirect sympathetic control of atrial stretch receptor discharge in the dog

Occurrence of long non-myelinated axonal segments intercalated in myelinated, presumably sensory axons: electron microscopic observations in the dog atrial endocardium

Electron microscopy of serial sections revealed the occurrence of long non-myelinated segments in myelinated, presumably sensory axons running in the left atrial endocardium of normal adult dogs. Four such non-myelinated segments were analysed in three myelinated axons. They varied from 20 to 150 microns in length, and differed from nodes of Ranvier in being invested by Schwann cells in the manner of unmyelinated nerve fibers. Short non-myelinated portions (20-25 microns long) were associated with a single Schwann cell, whereas the longest such segment (150 microns) had five. The non-myelinated axonal segments were non-varicose and similar in diameter (1.2-3.0 microns) to adjacent myelinated segments, which had myelin sheaths 6-25 lamellae thick. The cytoplasm of the non-myelinated axonal segments contained numerous neurofilaments and microtubules, some mitochondria and smooth endoplasmic reticulum. The short non-myelinated segments were enclosed by perineurium, whereas the long non-myelinated segment was devoid of perineurium at its mid-portion; instead fibroblast-like cells made a loose boundary around the axon at this level. The significance of these non-myelinated segments was discussed with special emphasis on the question of whether they result from focal degeneration of the myelin sheath (demyelination) or are generally present in the preterminal regions of some axons.

The mechanism of adaptation of left atrial stretch receptors in dogs with chronic congestive heart failure

Chronic congestive heart failure (CHF) was induced in dogs by the construction of an aorto-caval fistula below the level of the renal arteries. Aorto-caval fistula dogs showed signs of CHF which included ascites, hind limb edema, and pulmonary congestion. Ventricular catheterization indicated a significantly higher left ventricular end diastolic pressure and lower maximum velocity of left ventricular pressure development/left ventricular end diastolic pressure in CHF dogs when compared to sham-operated controls. Heart weight/body weight ratios were significantly higher in CHF dogs. Electrophysiological recordings from medullated left atrial type B receptors from the cervical vagus indicated a depressed sensitivity of these receptors in CHF dogs when compared to sham-operated control dogs. For any given change in left atrial pressure, the discharge of left atrial receptors was significantly reduced in CHF dogs compared with sham-operated controls. The mechanism for this depressed sensitivity was investigated. Sonomicrometry of the left atrial appendage indicated a decreased compliance of the left atrial appendage in the dogs with chronic CHF. In addition, microscope examination of the complex unencapsulated receptor endings taken from the left atrial endocardium indicated a marked alteration in receptor morphology. A loss of the end arborization was the most typical finding. It is concluded that chronic CHF brought about by an aorto-caval fistula results in a depressed left atrial stretch receptor response and that both decreased left atrial compliance and structural alterations in the receptor endings may account for this depressed sensitivity.

The contribution of atrial stretch receptors to salt and water homeostasis in the human

Studies carried out in both the human and experimental animals provide reasonably strong evidence that receptors contained within the atria are involved in the control of body water. When atrial pressure is increased (and the atria distended) atrial receptor discharge is increased leading to an increase in urine flow and, depending upon the experimental condition, a less consistent increase in sodium excretion. At least two mechanisms appear to contribute to the renal response; inhibition of the secretion of antidiuretic hormone and inhibition of renal nerve discharge. None of the factors presently known to alter sodium excretion have been shown to account completely for the increase in sodium excretion. The failure of patients with chronically distended atria to experience a chronic diuresis appears to be partly the result of a resetting of atrial receptor sensitivity. Recent evidence suggests that the sensitivity of atrial volume receptors has a species variation.