Allometry of left ventricular myocardial innervation

Abstract

Body mass (BM) of terrestrial mammalian species ranges from a few grams in the case of the Etruscan shrew to a few tonnes for an elephant. The mass-specific metabolic rate, as well as heart rate, decrease with increasing BM, whereas heart mass is proportional to BM. In the present study, we investigated the scaling behaviour of several compartments of the left ventricular myocardium, notably its innervation, capillaries and cardiomyocytes. Myocardial samples were taken from 10 mammalian species with BM between approximately 2 g and 900 kg. Samples were analysed by design-based stereology and electron microscopy and the resulting data were subjected to linear regression and correlation analyses. The total length of nerve fibres (axons) in the left ventricle increased from 0.017 km (0.020 km) in the shrew to 7237 km (13,938 km) in the horse. The innervation density was similar among species but the mean number of axons per nerve fibre profile increased with rising BM. The total length of capillaries increased from 0.119 km (shrew) to 10,897 km (horse). The volume of cardiomyocytes was 0.017 cm(3) in the shrew and 1818 cm(3) in the horse. Scaling of the data against BM indicated a higher degree of complexity of the axon tree in larger animals and an allometric relationship between total length of nerve fibres/axons and BM. In contrast, the density of nerve fibres is independent of BM. It seems that the structural components of the autonomic nervous system in the heart are related to BM and heart mass rather than to functional parameters such as metabolic rate.

Keywords: allometry; cardiac innervation; electron microscopy; stereology.

Figure 1

Illustration of the estimation of volumes and surface areas. A grid consisting of 12 test line segments is projected on an electron micrograph obtained by systematic uniform random sampling. The end points of the line segments are considered as test points. For volume estimation, points hitting structures of interest (here: mf, myofibrils, mi, mitochondria, cl, capillary lumen, sp, sarcoplasm) are counted and then related to all test points hitting the reference volume. For surface area estimation, intersections of the test line segments with the boundary of a structure of interest are counted (here: arrows indicate intersection of line segment with cardiomyocyte plasma membrane; asterisks indicate intersection of line segment with luminal plasma membrane of capillary endothelium) and related to the total length of test line hitting the reference volume.

Figure 2

Illustration of the estimation of length. An unbiased counting frame with an exclusion line (solid line) and an inclusion line (dashed line) is projected on an electron micrograph obtained by systematic uniform random sampling. Structures of interest [here: nerve fibres with their axons (ax) and Schwann cell processes (sc)] are counted if they lie entirely or partly within the counting frame area and do not touch the exclusion line or its extensions. The number of counted nerve fibre (axon) transects is later related to the total counting frame area hitting the reference volume. Here, two nerve fibre profiles with two axon profiles each are located between a cardiomyocyte and connective tissue fibrils. coll, collagen.

Figure 3

Allometric relationship between various cardiac parameters and body mass. Log–log plots including the exponent b are shown for total left ventricular axon length (a), mean number of axon profiles per nerve fibre profile (b), volume of cardiomyocytes (c), surface area of cardiomyocytes (d), luminal volume of capillaries (e) and luminal surface area of capillary endothelium (f) relative to body mass. See text for further details.