Contractility studies on isolated bovine choroidal small arteries: determination of the active and passive wall tension-internal circumference relation.

Studies on isolated choroidal arteries could help to understand the regulatory mechanisms in the choroidal circulation. The aim of the present study was therefore to assess whether contractility studies on isolated choroidal arteries were feasible and to determine the active and passive wall tension-internal circumference relation of these arteries. This relation is essential for reliable further pharmacodynamic studies on these vessels. Isolated choroidal arteries were mounted on a wire myograph for isometric tension recording. After the vessel was mounted, the L(100) (the circumference of the vessel at a transmural pressure of 100 mmHg) was determined. Then the passive and active wall tension-internal circumference relation of the choroidal vessels was obtained by stepwise increasing the internal circumference. The changes in the internal circumference were expressed as a percentage of L(100). After each increase in circumference, the passive tone (in a calcium free medium), the spontaneous tone (in a Krebs--Ringer bicarbonate solution) and the active tone (in a solution containing K(+) 120 mM and prostaglandin F(2 alpha) 30 microM) was measured. The passive tone of the vessel increased exponentially with the circumference of the vessel. Both the spontaneous tone and the active tone also increased when the vessel was stretched. They peaked when the internal circumference approached 90% of the L(100) and diminished again when the circumference was further increased. The peak value of the active tension curve averaged 2.24+/-0.47 Nm(-1) (n=10). The passive tension was 0.57+/-0.08 Nm(-1) (n=10) at this circumference. The peak value of the spontaneous tension curve averaged 0.37+/-0.08 Nm(-1) (n=10). It can be concluded that in vitro contractility studies on isolated choroidal arteries are feasible. The optimal length or preload of the choroidal arteries is attained when the internal circumference of the artery is set to 90% of the L(100).