Phillip M. Hutchins

Comparison of microvascular pressures in normal and spontaneously hypertensive rats

Abstract The principal cardiovascular manifestations of essential hypertension are known to be an increase in peripheral vascular resistance and in systemic arterial blood pressure. However, it is not known if the increased vascular resistance is of appropriate location and magnitude to prevent an elevation of pressures in the microvasculature during hypertension. To examine this possibility, the pressures and diameters of microvessels in the cremasteric muscle were measured in normal (NR) and spontaneously hypertensive rats (SHR). The pressures and diameters in comparable orders of vessels in NR and SHR at their respective systemic arterial pressures of 88.5 ± 2.1 and 115.6 ± 1.8 mm Hg at age 7–8 weeks were measured by the servonull micropipet technique. Both the systemic pressure and the microvascular pressure in SHR are 30–35% higher than those measured in NR. The diameters of the arterioles and venules in SHR are as large or larger than comparable orders of vessels in NR. Therefore, the reported increase in vascular resistance during hypertension seems not to be caused by vasoconstriction of vessels in SHR as compared to NR, nor does the increased resistance prevent an increase in microvascular pressures.

Hypoxia-induced angiogenesis in chick chorioallantoic membranes: a role for adenosine

The effect of a chronic hypoxic stimulus and of altered adenosine metabolism on vascular density was studied in the chick chorioallantoic membrane (CAM). Eggs were incubated in 15% oxygen/85% nitrogen for 3 or 7 days beginning at 7 days of age. Vessel density of the CAM was estimated by counting the number of vessels intersecting 4 concentric circles (72 mm total circumference) placed over the formalin-fixed membrane. The 15% oxygen stimulated 34-41% increases (P less than 0.001) in CAM vascularity after 3 or 7 days. Nitrobenzylthioinosine (NBTI), an adenosine re-uptake inhibitor, augmented the hypoxia-induced angiogenesis an additional 17% and 14% (P less than 0.001) at Days 10 and 14, respectively. Methyl-isobutylxanthine (MIX), an adenosine receptor blocker, reduced the vasoproliferation by 66% (P less than 0.001) at both times. Topically suffused adenosine elicited a local concentration-related increase in vascularity. This response was completely blocked by MIX. Exposure to 15% oxygen for 7 days stimulated a 13.7% increase in the hematocrit (P less than 0.001). Embryo weights were reduced 20.7% (P less than 0.001). These findings point to a modulatory role for adenosine in hypoxia-induced angiogenesis, and support the broader hypothesis that vasoactive metabolites produced in response to hypoxic conditions partially mediate a structurally based long-term autoregulatory response.

Arteriolar rarefaction in the conjunctiva of human essential hypertensives

Abstract Hypertension has been shown to have a component of increased vascular resistance. In general, the terms increased resistance and vasoconstriction have been used interchangeably. This study demonstrates that in the bulbar conjunctiva of human essential hypertensives, the decrease in number of arterioles may be just as significant as the arteriolar constriction. When compared to 12 age and sex matched normotensive controls, 12 untreated essential hypertensives had a 20% reduction in arteriolar density and a 5% decrease in arteriolar diameter. Both of these findings were significant at the 5% level. In a calculated “Poiseuille resistance,” the vessel rarefaction and the vessel constriction would contribute approximately equally to the increased resistance. These findings support the contention that microcirculatory density and caliber should be considered in discussions of alterations of vascular resistance.

The microcirculation in experimental hypertension and aging

OBJECTIVE The purpose of this manuscript is to review the literature concerning the alterations in the microvasculature in experimental hypertension and aging. We also present new unpublished data and results where previous studies have not addressed important questions. METHODS The new studies were performed using a chronic cranial window to allow multiple observations of the cortical surface vasculature over time. In vivo video, microscopic techniques were used to study long-term changes in microvascular caliber (vasomotion). In some studies, a chronic, in-dwelling aortic catheter allowed chaotic analysis of short-term blood pressure and heart rate variations. RESULTS In these new studies we demonstrated a reduction in number of small arteriolar endpoints per cortical surface area in the spontaneously hypertensive rat and in the old Brown-Norway rat. There was also a reduction in the number of arteriole-to-arteriole anastomotic connections in the older rat. These vascular changes in the old rat were revised or prevented by caloric restriction. In the old rat, there was also a reduction in the variability of blood pressure, heart rate and microvessel caliber (vasomotion). CONCLUSIONS These studies suggest that there is an alteration in the morphology of the small arterioles in hypertension and aging, that may lead to reduced ability to perfuse cortical tissue. In addition, there appears to be a diminution of overall short-term cardiovascular and microvascular control.

Pressor and cardioaccelerator effects of gamma MSH and related peptides

We have recently demonstrated that the hypertensinogenic and natriuretic actions of ACTHI-39 can be found in a non-steroidogenic fragment of ACTH, ACTH4-10. These effects of ACTH or ACTH4-10 may be due to their ability to act as weak agonists of gamma MSH. gamma MSH is found in the 16K N-terminus of pro-opiocortin, and contains a sequence analogous to ACTH4-10, gamma MSH3-9. We investigated the cardiovascular effects of gamma 2MSH, gamma MSH3-9, and sterically restricted analogs of ACTH4-10. The results indicate that gamma MSH3-9, had essentially the same activities as ACTH4-10. The addition of five other amino acid residues to gamma MSH3-9 (gamma 2MSH) resulted in significant enhancement of pressor and cardioaccelerator activity. Steric restriction of the ACTH4-10 sequence by the substitution of a D-Phe in place of an L-Phe residue in position #7, or cyclization of the peptide by a half-Cys4, half Cys10 intramolecular disulfide-bridge derivatization, resulted in increased cardiovascular activities. Based on these data, the cardiovascular actions of ACTH4-10, gamma MSH3-9, and gamma 2MSH are predicted to be due to the assumption of a reverse-turn three-dimensional structure. The additional residues in gamma 2MSH appear to specifically enhance the cardiovascular activities of gamma MSH3-9. The results suggest the existence of a new class of hypophyseal peptides with cardiovascular activities, which require the assumption of a defined three-dimensional structure.

Long-term micro- and macrocirculatory measurements in conscious rats

A lightweight, thermoneutral chamber design is introduced which permits the investigator to study, in vivo, intact skeletal muscle vasculature in the rat. The chamber, combined with established instrumentation (electromagnetic flowmetry and chronic catheterization) for monitoring of central hemodynamic parameters, affords the unique opportunity to correlate hemodynamic changes with alterations in the skeletal muscle microvasculature. Further, the studies may be performed in the unanesthetized animal. This model represents an integrated approach for the simultaneous study of the macro- and microcirculation within the same animal.

Enhanced vasomotion of cerebral arterioles in spontaneously hypertensive rats

Intrinsic rhythmic changes in the diameter of pial cerebral arterioles (30-70 microns) in anesthetized normotensive and hypertensive rats were assessed in vivo to determine if any significant differences exist between the two strains. All diameter measurements were analyzed using a traditional graphic analysis technique and a new frequency spectrum analysis technique known as the Prony Spectral Line Estimator. Graphic analysis of the data revealed that spontaneously hypertensive rats (SHR) possess a significantly greater fundamental frequency (5.57 +/- 0.28 cycles/min) of vasomotion compared to the control Wistar-Kyoto normotensive rats (WKY) (1.95 +/- 0.37 cycles/min). Furthermore, the SHR cerebral arterioles exhibited a significantly greater amplitude of vasomotion (10.07 +/- 0.70 microns) when compared to the WKY cerebral arterioles of the same diameter (8.10 +/- 0.70 microns). Diameter measurements processed with the Prony technique revealed that the fundamental frequency of vasomotion in SHR cerebral arterioles (6.14 +/- 0.39 cycles/min) was also significantly greater than that of the WKY cerebral arterioles (2.99 +/- 0.42 cycles/min). The mean amplitudes of vasomotion in the SHR and WKY strains obtained by the Prony analysis were found not to be statistically significant in contrast to the graphic analysis of the vasomotion amplitude of the arterioles. In addition, the Prony system was able to consistently uncover a very low frequency of vasomotion in both strains of rats that was typically less than 1 cycle/min and was not significantly different between the two strains. The amplitude of this slow frequency was also not significantly different between the two strains. The amplitude of the slow frequency of vasomotion (less than 1 cycle/min) was not different from the amplitude of the higher frequency (2-6 cycles/min) vasomotion by Prony or graphic analysis. These data suggest that a fundamental intrinsic defect exists in the spontaneously hypertensive rat that may contribute to the pathogenesis of hypertension in these animals.

Thymectomy delays the development of hypertension in Okamoto spontaneously hypertensive rats

Previous studies in rats have demonstrated that immune system dysfunction contributes to the aetiology of spontaneous hypertension. Chronic immunosuppression with cyclophosphamide attenuated the level of hypertension in Okamoto spontaneously hypertensive rats (SHR) by approximately 50%. Also, neonatal thymic implants delayed the development of spontaneous hypertension and significantly attenuated its level at the age of 22 weeks in SHR. In the present study, the effect of thymectomy at the age of 4 weeks on blood pressure was investigated in SHR and Wistar-Kyoto (WKY) rats. The removal of the thymus gland in 4-week-old SHR produced a significant reduction in systolic arterial pressure (SAP), diastolic arterial pressure (DAP) and mean arterial pressure (MAP) when rats were 16-19 weeks old, while no pressure reduction was observed in WKY rats. The decrease in arterial pressure of 16-week-old SHR was associated with a significant reduction in lymphocyte count at this age as compared with the control group. In 1-year-old SHR, thymectomized at the age of 4 weeks, there was no significant difference in arterial pressure or lymphocyte count compared with controls. These data support the hypothesis that an immune imbalance may be important in the development of spontaneous hypertension. We conclude that thymectomy at a young age (4 weeks) delays the development of hypertension in SHR.

Natriuretic and hypertensive activities reside in a fragment of ACTH.

The hypertensive and natriuretic effects of chronic administration of adrenocorticotrophic hormone (ACTH) cannot be duplicated by the administration of glucocorticoids and/or mineralocorticoids. We investigated the effects of a fragment of this hormone (ACTH4-10) and an analog of the fragment (D-Phe7) ACTH4-10 and found them to have pressor and cardioaccelerator actions in rats as determined by bolus intravenous (i.v.) injections of 30 to 1000 nmol/kg. The pressor and cardioaccelerator effects of (D-Phe7) ACTH4-10 were attenuated by alpha-receptor (phentolamine) and beta-receptor (metoprolol) antagonists. The cardiovascular actions of ACTH4-10 were produced in adrenalectomized or ganglionic-blocked (with mecamylamine) rats. At a lower dose (7 nmol/kg i.v.), ACTH4-10 was natriuretic and had a pattern of activity similar to that of a larger ACTH fragment, alpha-melanocyte-stimulating hormone. Extraadrenal effects of the intact ACTH molecule or the in vivo production of an ACTH4-10-like fragment from ACTH may contribute to the hypertensive and natriuretic actions associated with this hormone.

Microvascular responses to chronic hypoxia by the chick chorioallantoic membrane: A morphometric analysis

Numerous studies have demonstrated an increased capillarity in response to hypoxia in a variety of tissues. We studied the effects of hypoxia on the number and morphology of pre- and postcapillary vessels in the chick chorioallantoic membrane (CAM). Measurements of CAM microvessels were made from in vivo photographs after incubation in 15% oxygen (hypoxia) for 7 days (Days 7-14 of development). Quantitation of arteriolar and venular number, diameter, and length within defined areas was performed using a digitizing tablet with a 0.001-in. resolution, or 1.2 microns on photographs enlarged to 30x. The 15% oxygen environment produced a 54% increase in overall vessel density, with arterioles increasing 78% and venules 34%. The increases were primarily among vessels less than 10 microns in diameter. Moreover, the total vessel length per area of CAM also was increased by the 15% oxygen. Among vessels less than 80 microns in diameter, the low oxygen regimen stimulated a preferential increase in the number of arterioles, evidenced by a significant upward shift in the arteriole: venule ratio. Control vs 15% oxygen groups showed no statistical differences for the diameters and lengths of individual arterioles and venules. Thus, the observed increase in the total vessel length per area following 15% oxygen reflects the increased number of vessels. These data demonstrate that chronic exposure of the CAM to low oxygen stimulates an increase in the density of the pre- and postcapillary vessels which favors the arterioles.