Use of a new index to study relaxation in a vascular model of anaphylactic shock

We have reported increased smooth muscle shortening ability in ragweed pollen-sensitized saphenous vein (SSV). This may account for the vascular hyperreactivity of anaphylactic shock. We have now investigated relaxation in SSV. Because isotonic relaxation is load and initial contractile element length dependent, we developed an adjusted half-relaxation time index, which was independent of these variables. Muscle activation state was monitored by measuring maximum unloaded velocity. The relaxation index showed no difference between SSV and control saphenous vein after 2.5, 10, and 15 s of electrical stimulation; however, after 1 s of stimulation it was prolonged significantly in SSV. We concluded that the cross bridges activating early in contraction demonstrated prolonged relaxation. Activation state during muscle relaxation spontaneously increased toward the end of relaxation, coincident with a slowing in isotonic re-elongation rate. This was seen only in muscles relaxing from 15 s of stimulation. Our results indicate that 1) the relaxation properties of early cycling (1 s) cross bridges are altered after sensitization; and 2) toward the end of isotonic relaxation, cross-bridge cycling rate increases spontaneously, a phenomenon not previously reported. We speculate that the rapid re-elongation in late relaxation may reactivate muscle.