Rupture of coronary vasa vasorum as a trigger of acute myocardial infarction.

Some controversy has always existed regarding the presence and extent of the vasa vasorum--the nutrient vessels in the wall of the human aorta--in the coronary arteries. Now, cinemicrographic studies using silicone polymer injections in cleared human hearts have identified the vasa vasorum of coronary arteries, revealing evidence of neovascularization in the region of atherosclerotic plaques. These studies suggest an important role for the vasa vasorum in the pathogenesis of coronary atherosclerosis and its sequelae, especially intramural hemorrhage and vascular spasm. The wall of the human coronary artery in regions of atherosclerotic injury may be particularly rich in capillary vessels of the vasa vasorum. From this, the evidence suggests that with the morning increase in blood pressure, fragile neovascular structures of the vasa vasorum may be more prone to rupture and may be responsible, in part, for the circadian variation in myocardial infarction.

Angiogenic activity of adipose tissue.

Adipose tissue has been used to promote wound healing and to revascularize ischemic myocardium. We explored whether fat from various sources was angiogenic in the cornea. Rabbit subcutaneous and omental fat induced grossly visible neovascularization of all rabbit corneas studied, and at a similar rate and intensity. Neovascularization was not observed in any cornea following control implantation of liver or muscle. Neovascularization was blocked in all rabbits in which indomethacin was administered orally 3 days before implantation of fat and continued following implantation, suggesting that prostaglandins are associated with fat induced angiogenesis.

Plasma atrial natriuretic factor during chronic thoracic inferior vena caval constriction.

The effects of chronic constriction of the thoracic inferior vena cava (TIVCC) on plasma atrial natriuretic factor (pANF) were studied in conscious dogs (n = 5). TIVCC decreased left and right atrial pressure and led to a decrease in pANF concentration from 199 +/- 12 to 104 +/- 14 pg/ml while plasma renin and vasopressin concentrations increased. These hormonal changes were associated with a significant fall in sodium excretion to less than 5 meg/day. pANF remained suppressed during chronic TIVCC as the dogs expanded their extracellular fluid volume and developed ascites. Acute release of TIVCC resulted in abrupt increases in left and right atrial pressure but only a modest rise in pANF from 96 +/- 16 to 185 +/- 45 pg/ml. The magnitude of the rise in pANF (twofold) contrasted sharply with the eightfold increase in sodium excretion that occurred over the first 24 hours. Our data suggest that decrease in atrial pressure below normal results in a decline in pANF, which, acting in concert with the activated renin-angiotensin system and vasopressin, may contribute to sodium retention. On the other hand, during acute release of TIVCC, which markedly increased atrial pressure and sodium excretion, pANF only returned to control levels. These data suggest that ANF release may be attenuated during chronic reduction in atrial pressure and also raise a question concerning the magnitude of the primary role of ANF in this natriuretic response.

Stimulus-response curve of the renal baroreceptor: effect of converting enzyme inhibition and changes in salt intake.

We investigated the effect of converting enzyme inhibition (CEI) on the relationship between renal perfusion pressure (RPP) and steady-state plasma renin activity (PRA) in uninephrectomized conscious dogs on normal-salt (80 meq Na+/day) and low-salt (10 meq Na+/day) diets. Stimulus-response curves for the renal baroreceptor were determined by measuring the steady-state PRA while the RPP was lowered and then held constant by an inflatable cuff placed around the renal artery. On each diet the control stimulus-response curve can be described by two lines intersecting at a threshold pressure; in the higher pressure range PRA is relatively insensitive to changes in RPP, while in the lower pressure range PRA is very responsive to changes in RPP. On the normal-salt diet CEI significantly increases the sensitivity of PRA to RPP in the responsive range without affecting the threshold pressure itself or the values of PRA at pressures greater than the threshold pressure. On the low-salt diet CEI also increases the sensitivity of PRA to RPP significantly in the responsive range; we were unable to determine the effect of CEI on PRA at RPPs greater than the threshold pressure in the low-salt state because CEI causes a significant drop in blood pressure under these circumstances. The effect of CEI was significantly greater in the dogs on the low-salt diet than in the dogs on the normal-salt diet. Thus, CEI and salt depletion interact synergistically to increase the sensitivity of the renal baroreceptor only in the responsive range of the stimulus-response curve, i.e., at renal perfusion pressures below the threshold pressure.

Neovascularization and coronary atherosclerotic plaque: cinematographic localization and quantitative histologic analysis.

A new technique was developed for analyzing the neovascularization associated with coronary artery atherosclerosis: cinematography during silicone polymer injection of the coronary arteries of fixed and cleared human hearts, followed by histologic analysis in routine and 1-micron-thick, Epon-embedded sections. Twenty-two hearts obtained at autopsy were studied. On the basis of cinematographic findings, individual regions of the coronary arteries were classified as negative, positive, or abundantly positive for neovascularization. Positive and abundantly positive areas, which invariably occurred in segments exhibiting changes of atherosclerosis, contained numerous small vessels in the adventitia and outer media (4.7 +/- 1.5 and 9.8 +/- 1.3 [SE] vessel profiles/artery cross-section in positive and abundantly positive areas, versus 1.0 +/- 0.6 in negative regions). Abundantly positive areas, which occurred in coronary artery segments demonstrating the most extensive atherosclerotic change, contained numerous small vessels in the inner media or in the plaque itself. Some of these microvessels were in close proximity to mast cells, which represent potentially rich sources of mediators affecting vascular tone and permeability. Vessels were not observed in the inner media or in atherosclerotic plaque in areas designated either positive or negative by cinematography. These findings show how our approach can be used both to define the three-dimensional, in situ configuration of coronary artery neovascularization and to characterize the histology of this process in detail. They also confirm previous work indicating that areas of coronary arteries involved by atherosclerosis frequently exhibit extensive neovascularization.

Assessment of autonomic function in humans by heart rate spectral analysis.

Spectral analysis of spontaneous heart rate fluctuations were assessed by use of autonomic blocking agents and changes in posture. Low-frequency fluctuations (below 0.12 Hz) in the supine position are mediated entirely by the parasympathetic nervous system. On standing, the low-frequency fluctuations increase and are jointly mediated by the sympathetic and parasympathetic nervous systems. High-frequency fluctuations, at the respiratory frequency, are decreased by standing and are mediated solely by the parasympathetic system. Heart rate spectral analysis is a powerful noninvasive tool for quantifying autonomic nervous system activity.

Comparison of renin-specific IgG and antibody fragments in studies of blood pressure regulation.

The use of antirenin antibody and its fragments as specific tools for the analysis of the role of renin in blood pressure regulation is assessed in this study. Specific immunoglobulin G (IgG) or its antibody fragments (Fab) raised against purified canine renal renin were employed. These agents had no effect on blood pressure in the sodium-replete unanesthetized dog but induced systemic vasodepressor response (16.4 +/- 2 mmHg mean pressure) and suppression of plasma renin activity in sodium-deplete animals. Renin-specific IgG or Fab restored mean systemic pressure of acute renovascular hypertensive dogs to normotensive levels (132 +/- 2 to 90 +/- 5 mmHg) associated with inhibition of plasma renin activity (11.2 +/- 2 to 1.4 +/- 0.7 ng angiotension I X ml-1 X h-1). A linear relationship was demonstrated between the reduction in mean systemic pressure and fall in plasma renin activity (r = 0.87, P less than 0.005). The onset of action of both agents occurred within minutes. However, the peak effect of Fab was observed within 1-2 min, while that of IgG was delayed by approximately 20 min. The duration of action of Fab was short lived (about 30 min) whereas that for IgG was prolonged (up to 24 h). The results demonstrate the usefulness of renin-specific IgG and Fab as tools in physiological studies.

Modulation of renal baroreceptor function by catecholamines and salt intake in the conscious dog.

We have examined the relationship between plasma renin activity and renal perfusion pressure by determining the stimulus-response curve of the renal baroreceptor in conscious, uninephrectomized dogs, and the modulation induced by catecholamines and varying salt intake. Renal perfusion pressure was controlled by step-wise inflation of a constricting cuff previously implanted around the renal artery. The stimulus-response curve of the renal baroreceptor may be divided into two ranges: a relatively flat portion close to normal pressure (100 mmHg), and a much steeper section below a "threshold" pressure of 75-80 mmHg. Epinephrine (I.V. or I.R.) produced a parallel shift of the curve to the right, i.e., a smaller drop in pressure was needed to reach "threshold." Changing the diet from normal salt intake (80 meq/day) to a low salt intake (10 meq/day) increased the gain of the stimulus-response curve. Thus, the same elevated PRA may be reached either by a shift of the curve to the right, or by increase in gain.

Is renin a factor in the etiology of essential hypertension?

The widespread clinical study of converting-enzyme inhibitors has shown that they are effective antihypertensive drugs even in patients who may manifest either normal or decreased plasma renin activity. This suggests either that renin in a site other than plasma may play a contributory role in essential hypertension or that the hypotensive effect is caused by increased concentrations of kinins and prostaglandins, both demonstrated consequences of converting-enzyme inhibitor administration. Specific renin inhibitors appropriate for studies in humans would aid in the resolution of this question. Four classes of compounds have been demonstrated to be renin inhibitors of high potency: specific antibody, general peptide inhibitors of acid proteases, analogs of angiotensinogens, and peptides that are related to the amino-terminal sequence of prorenin. With the purification of renin, specific polyclonal or monoclonal antibodies have become available. The former have already been used extensively in physiologic studies in intact animals. Pepstatin is an inhibitor of many acid proteases. Its in vivo application has been retarded by its relative insolubility, but recent chemical modifications, particularly the addition of charged amino acids at the carboxy terminus, have rendered it more useful. The minimal substrate for renin is an octapeptide segment of the protein substrate: His-Pro-Phe-His-Leu-Leu-Val-Tyr. Variants of this sequence have resulted in competitive inhibitors that are useful in vivo. Recently, remarkably active inhibitors have been synthesized by reducing the peptide bond that is cleaved by renin, producing what may be a transition state inhibitor.(ABSTRACT TRUNCATED AT 250 WORDS)

Alteration of renal baroreceptor by salt intake in control of plasma renin activity in conscious dogs.

We investigated the relationship between renal arterial pressure (RAP) and systemic plasma renin activity (PRA) in five uninephrectomized conscious dogs on normal salt (80 meq Na+/day) and low salt (10 meq Na+/day) diets. The RAP was controlled by an inflatable cuff placed around the origin of the renal artery. In both salt states the PRA was an exponential function of the RAP: log (PRA) = (-0.026 X RAP) + 2 on the normal salt diet (r = 0.96) and log (PRA) = (-0.026 X RAP) + 2.5 on the low salt diet (r = 0.99). At any RAP, the value of the low salt PRA was 3 times that of the normal salt PRA. Accordingly, a reduction in salt intake increases the sensitivity of the renal baroreceptor so that the absolute value of PRA increases at any RAP, but the percentage change in PRA caused by any change in RAP is the same in both normal and low salt states.

Compensatory response to hemorrhage in conscious dogs on normal and low salt intake.

The compensatory response to moderately severe hemorrhage (30 ml/kg) was studied in chronically catheterized conscious dogs maintained on normal and low salt intake. Although the fall in blood pressure and the increase in heart rate were similar in the two salt states, the salt-restricted animals had significantly greater rises in plasma renin activity and plasma catecholamines following hemorrhage than did the normal salt dogs. To compare further the relative roles of the alpha-adrenergic system and the renin-angiotensin system in the maintenance of blood pressure following hemorrhage, pharmacologic blockade with either phentolamine or converting enzyme inhibitor was performed 20 min after the completion of the hemorrhage. These latter experiments demonstrated that salt restriction resulted in a significantly greater role for the renin-angiotensin system. Moreover, interruption of the renin-angiotensin system blunted the anticipated rise in catecholamines and heart rate during the additional hypotension induced by converting enzyme blockade after hemorrhage.

Effect of intrarenal administration of dopamine on renin release in conscious dogs.

We investigated the effect of intrarenal administration of dopamine on renin release in conscious dogs. Dopamine in doses ranging from 0.28 to 3.0 micrograms . kg(-1) . min(-1) produced a significant increase in systemic plasma renin activity (PRA) and renin secretion rate without altering systemic blood pressure. Dopamine also induced renal vasodilatation and natriuresis within this dose range. To determine if the dopamine-induced renin release is related to its vasodilatory action, two other vasodilators, papaverine and acetylcholine, were infused into the renal artery, but neither, in doses that produced a rise in renal blood flow similar to that of dopamine, had any effect on PRA. As dopamine can activate alpha- and beta-adrenergic receptors in addition to dopaminergic receptors, experiments were also performed to characterize the type of receptors involved in dopamine-induced renin release. Intrarenal infusion of sulpiride and haloperidol, dopamine antagonists, significantly inhibited dopamine-induced renin release and renal vasodilatation. In contrast, intrarenal infusion of propranolol failed to alter dopamine-induced rise in PRA or renal blood flow. Simultaneous infusion of phentolamine and dopamine, on the other hand, produced a significant potentiation of dopamine-induced renin release and renal vasodilatation. In conclusion, our studies demonstrate that dopamine is capable of inducing renin release and renal vasodilatation in conscious dogs. Moreover, such actions of dopamine are mediated through activation of specific dopamine receptors in the kidney. Finally, we present evidence for the existence of the intrarenal alpha-adrenergic mechanism that is inhibitory to renin release.

Antibodies as specific renin inhibitors: studies with polyclonal and monoclonal antibodies and Fab fragments.

The use of antirenin antibody and Fab may provide a more specific physiologic tool and potential therapeutic agent than the existing pharmacologic inhibitors. The antibody combining site, by virtue of its larger size than organic compounds, has the capacity for a larger number of intramolecular contacts with its ligands, thus allowing increased selectivity and affinity. Renin-specific Fab obtained through immunization with purified dog renal renin has been studied. Fab had no effect on blood pressure in the sodium replete conscious dog but induced systemic hypotensive responses in the sodium deplete animal or during acute renovascular hypertension. These responses were accompanied with complete suppression of plasma renin and angiotensin II levels. The vasodepressor responses of Fab were comparable to the converting enzyme inhibitor, teprotide. However, the latter induced a greater renal vasodilator effect. Since antibodies and their fragments derived from immune sera are limited in their application to physiologic study with respect to lack of homogeneity, reproducibility and limitation of quantity, we obtained monoclonal antibodies to purified human renal renin. Fusion HR3 yielded 14 renin-antibody secreting clones. Six clones have been isolated and characterized. Isotypes include IgG1 and IgM. Kd's range from 6 X 10(-9) to 7.5 X 10(-10)M renin concentration. Five clones produce antibodies with antienzymatic activity. IC50's of these antibodies range from 1 X 10(-6) to 6 X 10(-8)M. All antibodies are renin-specific and do not bind several nonrenin enzymes and proteins examined. They are also highly species specific, i.e., do not crossreact with mouse, dog, bovine or rat renins but recognize hog renin. These catalytic-site directed antibodies may be highly potent and specific tools for physiologic studies in subhuman primates and man.