Functional study on the effects of nifedipine, cromakalim, and the absence of extracellular Ca2+on α1-adrenoceptor-mediated excitation-contraction coupling in isolated rat portal vein: Comparison with depolarization-mediated excitation-contraction coupling

The effects of Ca2+-entry blockade by nifedipine, K+channel opening by cromakalim, and of omitting extracellular Ca2+on the contractile response elicited by a supramaximum concentration of the 'full' and selective α1- adrenoceptor agonist phenylephrine (10-4M) were compared with those elicited by a supramaximal concentration of KCl (50 mM) in isolated rat portal vein. The contractile response to phenylephrine appeared to be biphasically composed of an early 'transient' phase and a slowly developing 'sustained' phase that reached maximum values after 30 s and 5 min after initiation of contraction, respectively. The contractile response to KCl (50 mM) exhibited a triphasic pattern consisting of 'spike,' 'transient', and 'sustained' components that peaked after 8 s, 25 s, and 10 min, respectively. Nifedipine was able to eliminate all components of the contractions in response to both phenylephrine and KCl almost completely. Nifedipine was ~10 times more potent at suppressing the slowly developing sustained components of the contractions in response to both stimuli than the early transient components. The spontaneous myogenic contractions were inhibited by nifedipine with intermediate potency. Cromakalim, in contrast to nifedipine, selectively eliminated the early transient components of the contractions in response to both phenylephrine and KCl. The sustained components of the contractions in response to both stimuli were relatively resistant to K+channel opening, although higher concentrations (>1 μM) of cromakalim were capable of antagonizing the sustained response to phenylephrine accompanied by oscillations in tone. Cromakalim was most potent in counteracting spontaneous myogenic contractions. When phenylephrine and KCl were added with or without external Ca2+after different periods of equilibration in nominally Ca2+-free medium, different washout kinetics for the different components of the contractions in response to both stimuli were observed. The early transient phases of tension development in response to both stimuli were completely lost after ~6 min of equilibration in nominally Ca2+-free medium, whereas the slowly developing sustained components of the contractions were immediately lost after the change to nominally Ca2+-free medium. Externally added Ca2+, when administered together with phenylephrine or KCl after the preparations had been exposed for different times to nominally Ca2+-free medium, could not restore the early transient components. In isolated rat portal vein, apart from absolute differences in height and time course of tension development which are probably caused by a difference in release of intracellular second messengers during the two responses, the contraction mediated by strong α1-adrenoceptor stimulation could, as tested, hardly be distinguished from the one produced by a maximum effective concentration of KCl. As for KCl stimulation, the Ca2+influx essential for contraction during the entire response to supramaximal α1- adrenoceptor stimulation can be almost completely, though differentially, inhibited by nifedipine or short periods of absence of extracellular Ca2+. For nifedipine, this differential inhibition probably reflects the voltage and time dependency of the interaction of this 1,4-dihydropyridine with the L-type Ca2+channel. As reflected by its slower washout kinetics, the transient component of the contraction in response to phenylephrine is, apart from Ca2+influx, also caused by release of intracellularly stored Ca2+. This concept is strengthened first because the transient component was selectively inhibited by cromakalim, a compound which was recently shown to deplete intracellular Ca2+stores by an unknown mechanism and second because this response cannot be immediately restored by readdition of extracellular Ca2+after depletion.