The Distensibility of the Resistance Vessels in Spontaneously Hypertensive Rats (SHR) as Compared with Normotensive Control Rats (NCR)

Abstract

Earlier studies on human essential hypertension, as well as on SHR, suggest that an increased media thickness, partly encroaching upon the vascular lumen, forms the main background of the increased flow resistance in primary hypertension (e.g. Folkow et al. 1970). In the present investigation a comparison was made concerning the distensibility of the resistance vessels in order to further explore the proposed difference in vascular design. The simultaneously perfused hindquarter vessels of SHR and NCR were exposed to sudden, repeated shifts of pressure and flow, first at maximal vasodilatation and then at stable levels of smooth muscle tone, induced by barium ions or noradrenaline. Relative changes of vascular internal radius (ri) were calculated from the pressure‐flow values, assuming ri to be proportional to 4 flow conductance. For given changes in distending pressure (Pd), the SHR resistance vessels exhibited considerably smaller percentual ri changes than the NCR ones, whether starting from the same Pd, the same ri, or even when both Pd and ri were initially kept identical in NCR and SHR by exposing the SHR vessels to lower concentrations of constrictor agents. This evidence of reduced vascular distensibility in SHR was obvious both at maximal dilation and, particularly, when smooth muscle tone was increased. This strongly supports the view that especially the media component of the SHR resistance vessels is enhanced in bulk, thus markedly influencing both “active” and “passive” resistance changes.

Report of the Hypertension Task Force of the National Heart, Lung, and Blood Institute. Current research and recommendations from the Subgroup on Local Hemodynamics.

“…Several investigators"-" B have found right ventricular hypertrophy in rats with various forms of experimental hypertension, suggesting the presence of elevated right ventricular pressure or flow loads. One group,* 40 however, found that normal right ventricular systolic pressure accompanied right ventricular hypertrophy in pentobarbital anesthetized SHR; this group also reported abnormalities in pulmonary vascular responses to locally administered angiotensin II and prostaglandin F to . Pulmonary artery wall thickness was reported normal unless the rats were subjected to hypoxic stress, at which time arterial hypertrophy was observed in SHR but not in WKY.…”

“…There is also evidence for reduced arterial distensibility in spontaneously hypertensive rats (SHR). 40 Vascular fibrosis and/or edema may contribute to decreased arterial compliance, but the mechanisms of altered compliance are not fully elucidated.…”

“…It has also been suggested that decreased compliance of the aorta and carotid arteries may contribute to resetting of the aortic and carotid baroreceptors. 40 '" There is need for further study of the mechanisms of the decreased arterial compliance in hypertension and of the functional implications of this decreased compliance, particularly in relation to its effect on baroreceptor function.…”

“…40 Relative changes in mean internal radius were calculated using the fourth power rule. Results were interpreted to indicate decreased arteriolar distensibility in SHR, due to hypertrophy of vascular smooth muscle.…”

“…Many studies of responses of microvessels to vasoactive agents have appeared; a small sample includes studies of Hallback et al, 40 Altura, 141 Messina et al, 142 and Miller and Harris. 148 Unfortunately, the data have not been quantified satisfactorily in terms of local resistances in the networks examined.…”