American Physiological Society

The hemodynamic determinants of myocardial oxygen utilization were ascertained in the isolated, metabolically supported, nonfailing canine heart. The primary determinant was found to be the total tension developed by the myocardium as indicated by the area beneath the systolic pressure curve (Tension-Time Index). The significance of these findings for the understanding of the low efficiency of the failing heart and the consequently increased importance of Laplace's law are discussed.

Morphological features of blood capillaries from various vertebrate forms and organs are presented. Simple classifications are proposed, based on presence or absence of continuous basement membrane, on the nature of the endothelial cell and on the presence or absence of a complete investment of pericapillary cells. It is suggested that these varying capillary structural features may be relevant to problems relating to exchange of materials between blood plasma and parenchymal cells. Simple three-digit notation systems are presented for characterizing and designating the type of any capillary with respect to the classifications submitted.

The percentages of 22 14C-labeled amino acids remaining in rat brain 15 s after carotid injection were measured relative to a simultaneously injected diffusible internal standard, 3HOH. The injected solution also contained a nondiffusible internal standard, [113mIn]EDTA to correct for incomplete brain blood compartment washout. Self-inhibition and cross-inhibition was demonstrated by inclusion of unlabeled amino acids and carboxylic acids. All amino acids tested, excluding proline, alanine, and glycine, could be assigned to one, and only one, blood-brain barrier carrier system. The neutral carrier system transported phenylalanine, leucine, tyrosine, isoleucine, methionine, tryptophane, valine, DOPA, cysteine, histidine, threonine, glutamine, asparagine, and serine. Affinity for a basic amino acid carrier system was demonstrated for arginine, ornithine, and lysine. A third, low-capacity independent carrier system transporting aspartic and glutamic acids was demonstrated.