Kinetics of contraction in depolarized smooth muscle from guinea-pig taenia coil after photodestruction of nifedipine

The time course and kinetics of force development following activation by o pening of L-type Ca2+ channels was investigated using photodestruction of t he Ca2+ channel blocker nifedipine in smooth muscle from the guinea-pig tae nia coli. 2. In muscles activated using high K+ and Ca2+ and subsequently inhibited w ith nifedipine, photodestruction of the drug using a strong ultraviolet lig ht flash initiated a rapid contraction. The force initiated by photodestruc tion of nifedipine reached near-maximal levels. This procedure eliminates d iffusional delays and can thus be used to investigate the kinetics of depol arization-induced contractions. 3. The rate of force development of contractions initiated by photodestruct ion of nifedipine was slower than that observed in maximally thiophosphoryl ated skinned fibres. This suggests the rate of force development is limited by activation steps in the activation cascade prior to the force generatio n of the cross-bridge system. 4. The rate of force development and the plateau force were dependent on th e extracellular [CaCl2] suggesting that the intracellular [Ca2+] determines the rate of phosphorylation and force development. The delay between illum ination and increase in force was about 300 ms. The delay was similar at lo w and high extracellular [CaCl2] indicating that buffering by superficial s arcoplasmatic reticulum does not introduce a delay in force development fol lowing activation of Ca2+ channels in this muscle.