Tonic force maintenance with reduced shortening velocity in arterial smooth muscle

The isotonic shortening velocity of swine carotid media tissues contracting in response to high K+, histamine, norepinephrine, or AC electrical stimulation rapidly increased to a maximum value and then declined to a steady-state level while force was still increasing or steady. The maximum shortening velocity calculated for no external load on the tissue (Vo) also decreased during the course of contractions when active stress remained constant. The fall in velocity with time was not Ca2+ dependent, because reductions in the [Ca2+] in high K+ solutions that significantly reduced the maximum stress (Fo) had no effect on Vo in the steady state. On washout of high K+ solutions, the ability of the tissue to shorten on isotonic quick release fell rapidly to low levels before isometric stress exhibited significant declines. The data indicate that cross-bridge cycling rates are modulated in this tissue. We suggest that this reflects the formation of attached, noncycling cross bridges (termed latch bridges), which constitute an internal load on the contractile system during tonic contractions.