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

The time course and kinetics of force development following activation by opening of L-type Ca2+ channels was investigated using photodestruction of the Ca2+ channel blocker nifedipine in smooth muscle from the guinea-pig taenia coli.In muscles activated using high K+ and Ca2+ and subsequently inhibited with nifedipine, photodestruction of the drug using a strong ultraviolet light flash initiated a rapid contraction. The force initiated by photodestruction of nifedipine reached near-maximal levels. This procedure eliminates diffusional delays and can thus be used to investigate the kinetics of depolarization-induced contractions.The rate of force development of contractions initiated by photodestruction of nifedipine was slower than that observed in maximally thiophosphorylated skinned fibres. This suggests the rate of force development is limited by activation steps in the activation cascade prior to the force generation of the cross-bridge system.The rate of force development and the plateau force were dependent on the extracellular [CaCl2] suggesting that the intracellular [Ca2+] determines the rate of phosphorylation and force development. The delay between illumination and increase in force was about 300 ms. The delay was similar at low and high extracellular [CaCl2] indicating that buffering by superficial sarcoplasmatic reticulum does not introduce a delay in force development following activation of Ca2+ channels in this muscle

Role of the purinergic and noradrenergic components in the potentiation by endothelin-1 of the sympathetic contraction of the rabbit central ear artery during cooling

1. To examine the role of the purinergic and noradrenergic components in the potentiation of endothelin-1 on the vascular response to sympathetic nerve stimulation, we recorded the isometric response of isolated segments, 2 mm long, from the rabbit central ear artery to electrical field stimulation (1–8 Hz) under different conditions, at 37°C and during cooling (30°C). 2. Electrical field stimulation produced frequency-dependent contraction, which was reduced during cooling (about 60% for 8 Hz). Both at 37°C and 30°C, phentolamine (1 μM) or blockade of α(1)-adrenoceptors with prazosin (1 μM) reduced, whereas blockade of α(2)-adrenoceptors with yohimbine (1 μM) increased, the contraction to electrical field stimulation. This contraction was increased after desensitization of P2-receptors with α,β-methylene adenosine 5′-triphosphate (α,β-meATP, 3 μM) at 37°C but not at 30°C, and was not modified by blockade of P2-receptors with pyridoxalphosphate-6-azophenyl-2,4′-disulphonic acid (PPADS, 30 μM) at either temperature. 3. Endothelin-1 (1, 3 and 10 nM) at 37°C did not affect, but at 30°C it potentiated in a concentration-dependent manner the contraction to electrical field stimulation (from 28±6 to 134±22%, for 8 Hz). At 37°C, endothelin-1 in the presence of phentolamine or prazosin, but not in that of yohimbine, α,β-meATP or PPADS, potentiated the contraction to electrical stimulation. At 30°C, phentolamine or yohimbine reduced, prazosin or PPADS did not modify and α,β-meATP slightly increased the potentiation by endothelin-1 of the response to electrical stimulation. 4. The arterial contraction to ATP (2 mM) and the α(2)-adrenoceptor agonist BHT-920 (10 μM), but not that to (−)-noradrenaline (1 μM), was potentiated by endothelin-1 at both 37°C and 30°C. 5. These results in the rabbit central ear artery suggest that the sympathetic response: (a) at 37°C, could be mediated mainly by activation of α(1)-adrenoceptors, with low participation of P2-receptors, (b) is diminished during cooling, probably by a reduction in the participation of α(1)-adrenoceptors, and in this condition the response could be mediated in part by P2-receptors, and (c) is potentiated by endothelin-1 during cooling, probably by increasing the response of both postjunctional α(2)-adrenoceptors and P2-receptors