STRUCTURE AND MECHANICS OF GROWING ARTERIAL MICROVESSELS FROM HYPERTROPHIED URINARY-BLADDER IN THE RAT

Rat bladder hypertrophy, induced by a partial ligation of the urethra, was used to study the accompanying changes of microvascular smooth muscle mechanics, pharmacology and morphology. A segment of a microarterial vessel to the bladder was taken from a defined anatomical location and studied in a wire myograph in vitro at the length for maximal isometric force development (L(max)). After 10 days of ligation, bladder hypertrophy resulted in a microvascular growth response compared to non-operated controls which was characterized by (i) an increase of the calculated diameter at L(max) from 134 +/- 5 mu m to 222 +/- 19 mu m; (ii) an increase of the media thickness from 22.4 +/- 19 mu m to 32.2 +/- 3.0 mu m; (iii) an increase of the active tension from 1.42 +/- 0.28 mN/mm to 3.06 +/- 0.33 mN/mm; (iv) no change of the wall/lumen ratio (from 0.83 +/- 0.10 to 0.79 +/- 0.15). Normalized length/force relations (active, passive and total) did not differ significantly between microarteries from control and hypertrophic bladders. Microvascular smooth muscle growth was also associated with a decreased sensitivity to K+-induced depolarization and an increased sensitivity to alpha(1)-adrenergic stimulation. No differences were noted regarding the Ca2+ sensitivity of force during K+-induced depolarization. The results suggest that microvascular growth (1) is immediately and positively influenced by the organ growth; (2) results in a functional resetting of the microvascular segments towards larger diameters without gross morphological or mechanical alterations; and (3) is accompanied by pharmacological alterations of the smooth muscle reactivity.