No abstract

We examined the ability of individual regions of the canine left ventricle to increase blood flow relative to baseline rates of perfusion. Regional coronary flow was measured by injecting radioactive microspheres over 90 seconds in seven anesthetized mongrel dogs. Preliminary experiments demonstrated a correlation between the regional distributions of blood flow during asphyxia and pharmacological vasodilatation with adenosine (mean r = 0.75; 192 regions in each of two dogs), both of which resulted in increased coronary flow. Subsequent experiments, during which coronary perfusion pressure was held constant at 80 mm Hg, examined the pattern of blood flow in 384 regions (mean weight, 106 mg) of the left ventricular free wall during resting flow and during maximal coronary flow effected by intracoronary adenosine infusion. We found that resting and maximal flow patterns were completely uncorrelated to each other in a given dog (mean r = 0.06, p = NS; n = 3 dogs). Furthermore, regional coronary reserve, defined as the ratio of maximal to resting flow, ranged from 1.75 (i.e., resting flow was 57% of maximum) to 21.9 (resting flow was 4.5% of maximum). Thus, coronary reserve is spatially heterogeneous and determined by two distinct perfusion patterns: the resting (control) pattern and the maximal perfusion pattern. Normal hearts, therefore, contain small regions that may be relatively more vulnerable to ischemia. This may explain the patchy nature of infarction with hypoxia and at reduced perfusion pressures as well as the difficulty of using global parameters to predict regional ischemia. Despite the wide dispersion of coronary reserve, we found, by autocorrelation analysis, that reserve in neighboring regions (even when separated by a distance of several tissue samples) was significantly correlated. This also applied to patterns of resting myocardial flow. Thus, both resting coronary blood flow and reserve appear to be locally continuous and may define functional zones of vascular control and vulnerability, respectively.

To examine the influence of cardiac contraction on systolic coronary flow and transmural blood flow distribution, we measured phasic blood flow velocity in distal extramural coronary arteries by Doppler velocimeter and regional myocardial blood flow by radiolabeled microspheres while the heart was beating and during prolonged diastoles in 12 dogs. A servo-controlled coronary perfusion circuit allowed mean coronary pressure to be selected and maintained during beating and diastolic conditions. In epicardial arteries just proximal to their entrance into the myocardium, blood flow was either negligible or reverse in direction during systole. When the heart was beating, subepicardial blood flow was 24.2 +/- 12.3% higher than during asystole (5.05 +/- 0.91 and 4.11 +/- 0.79 ml.min-1.g-1 for beating and prolonged diastoles, respectively; P less than 0.01). In the subendocardium, flow was 49.8 +/- 14.7% lower in the beating condition than during prolonged diastoles (4.23 +/- 1.46 and 8.26 +/- 1.71 ml.min-1.g-1 for beating and asystole, respectively; P less than 0.01). When heart rate was increased stepwise from 60 to 150 beats/min, subendocardial flow fell approximately linearly; flow to the superficial layer was relatively unaffected. In beating hearts, lowering mean left main coronary artery (LMCA) pressure from 80 to 50 mmHg resulted in more systolic reverse flow and a fall in inner-to-outer flow ratio from 0.82 +/- 0.18 to 0.66 +/- 0.34 (P less than 0.05). Because mean LMCA pressure was held constant when the heart was stopped, differences in regional blood flow between beating and diastolic conditions were primarily due to cardiac contraction. Because little or no blood entered the myocardium from the extramural arteries during systole, we conclude that the decrease in subendocardial flow and the increase in subepicardial flow were caused by retrograde pumping of blood from the deep layer to the superficial layer of the left ventricle. Systolic retrograde flow to the subepicardium may help explain this layer's relative protection from ischemia.

The bicyclic diacid 1 was designed as a semi-rigid template for the hydrogen-bonding pattern of a peptide R-helix. The protected precursor 7 was synthesized in eight steps from tert-butyl 3,5-dimethoxybenzoate and linked to L-alanine and L-lactic acid to provide derivatives appropriate for coupling to a peptide. Both the amide 8-N and the ester 12-O were obtained in each of the four diastereomeric forms. The structure of R,R-8-N was determined by X-ray crystallography, which facilitated assignment of the diastereomers and confirmed the intended conformational effects of the quaternary methyl groups. The bicyclic amide and ester derivatives were appended to the peptide EALAKA-NH 2 , and their influence on the conformation was evaluated in aqueous solution using circular dichroism and NMR. The amide analogs have only a slight effect on the appended peptide, whereas the ester-linked template in S,S-9-O induces 32-50% helical character at 23°C and 49-77% at 0°C, depending on the method of determination; significant helical character persists even at 70°C. The ability of the template 1 to induce the helical conformation is related to its structural and electronic complementarity to the N-terminus of the peptide; templating ability disappears when the carboxylate in S,S-9-O is protonated, and it is not observed in the dimethylamide S,S-9-N-a. The structural and dynamical properties of conjugate S,S-9-O were studied by NMR and compared with those of the acetylated heptapeptide 13. The dispersion and temperature dependence of amide hydrogen chemical shifts and the pattern observed for intra-and inter-residue nuclear Overhauser enhancements are all consistent with a significant population of helical conformers within the conformational ensemble of conjugate S,S-9-O, in contrast to the unstructured peptide 13. The generalized order parameter S 2 was derived for each residue from the 15 N T 1 and T 2 relaxation rate constants and 1 H-15 N heteronuclear NOEs determined for the 15 N-labeled derivative of S,S-9-O; these parameters demonstrate a high degree of conformational rigidity along the peptide chain at 4°C, with relative motion increasing for the C-terminal residues. These data are consistent with the chiroptical studies and demonstrate that the template is exceptionally effective in inducing helical behavior in an appended peptide.