Allometry of left ventricular myocardial innervation

Area of Science:

• Comparative physiology
• Mammalian biology
• Cardiovascular anatomy

Background:

• Mammalian body mass (BM) varies widely, influencing physiological parameters like metabolic rate and heart rate.
• Heart mass scales proportionally with BM, but the structural adaptations of the heart's components across this range are less understood.

Purpose of the Study:

• To investigate the scaling relationships of left ventricular myocardial compartments, including innervation, capillaries, and cardiomyocytes, with body mass in mammals.
• To determine if these structural components scale with BM or functional parameters like metabolic rate.

Main Methods:

• Stereology and electron microscopy were used to analyze myocardial samples from 10 mammalian species (2 g to 900 kg).
• Linear regression and correlation analyses were performed on quantitative data regarding nerve fiber length, capillary length, and cardiomyocyte volume.

Main Results:

• Total nerve fiber (axon) length and capillary length increased significantly with BM, showing an allometric relationship.
• Innervation density was consistent across species, but the number of axons per nerve fiber profile increased with BM.
• Cardiomyocyte volume also scaled with BM.

Conclusions:

• The complexity of the cardiac axon tree increases with BM.
• Structural components of the heart's autonomic nervous system and vasculature scale primarily with BM and heart mass, rather than functional parameters such as metabolic rate.