Potential-dependent interactions of nitrendipine and related 1,4-dihydropyridines in functional smooth muscle preparations

Comparative studies of AJG049, a novel Ca2+ channel antagonist, on voltage-dependent L-type Ca2+ currents in intestinal and vascular smooth muscle

BACKGROUND AND PURPOSE

Antagonists of Ca2+ channels reduce contraction of intestinal smooth muscle but also affect vascular smooth muscle. We have therefore examined the effects of AJG049, a newly synthesized antagonist for regulation of gut motility, on voltage-dependent L-type Ca2+ channels, in vascular and intestinal smooth muscle, comparing AJG049 with two other Ca2+ channel antagonists, verapamil and diltiazem.

EXPERIMENTAL APPROACH

Affinities of AJG049 for various types of voltage-dependent Ca2+ channels were examined by binding studies. Effects of AJG049 on voltage-dependent inward Ca2+ (or Ba2+) currents (ICa or IBa) in dispersed smooth muscle cells from guinea-pig ileum, colon and mesenteric artery were measured using conventional whole-cell configurations.

KEY RESULTS

In binding studies, AJG049 showed a high affinity for the diltiazem-binding site of L-type Ca2+ channels. In whole-cell configuration, AJG049 suppressed ICa in ileal myocytes, with concentration-, voltage-and use-dependencies. AJG049 shifted the steady-state inactivation curve of ICa to the left. The order of potency to inhibit ICa in ileal myocytes was AJG049>verapamil>diltiazem. AJG049 also suppressed IBa in guinea-pig mesenteric arterial myocytes, showing concentration- and voltage-dependencies and the potency order for this action was also AJG049>verapamil>diltiazem. For the relative ratio of Ki values between ileal and mesenteric arterial myocytes, the order was AJG049>diltiazem>>verapamil.

CONCLUSIONS AND IMPLICATIONS

These results show that AJG049 inhibits L-type Ca2+ channels mainly through the diltiazem-binding site(s). From our results, AJG049 showed a little selectivity for these Ca2+ channels in intestinal smooth muscle.

State-dependent inhibition of L-type calcium channels: cell-based assay in high-throughput format

The current studies describe a new, robust cell-based functional assay useful to characterize L-type voltage-dependent calcium channels and their antagonists. The basis of this assay is measurement in plate format of Ca2+ influx through the L-type Ca2+ channel complex (alpha1C, alpha2delta, and beta2a subunits) in response to potassium-mediated depolarization; EC(50)=11 mM [K+](o). The Ca2+ influx was inhibited by the L-type Ca2+ channel antagonist, nimodipine; IC(50)=59 nM. These cells were also transfected with the Kir2.3 inward rectifier K(+) channel, which allows for changing the cell membrane potential by modulation of extracellular [K](o); -65 mV in physiological [K](o) and -28 mV in 30 mM [K](o) containing buffer. The conformational state of the voltage-sensitive Ca2+ channel is altered under these different conditions. Under the depolarized condition, nimodipine was a more potent antagonist, inhibiting Ca2+ influx with an IC(50) value of 3 nM. The results demonstrate that the interaction of nimodipine and other antagonists with the channel is modulated by changes in membrane potential and thus the state of the channel. Overall, this novel assay can be used to identify state-dependent calcium channel antagonists and should be useful for evaluating state-dependent inhibitory potency of a large number of samples.

Effect of divalent cations on KCl- and noradrenaline-induced contractile responses in rat aorta after nifedipine treatment

Nifedipine (1 microM) relaxed the sustained contractile responses induced by 1 microM noradrenaline or 60 mM KCl in rat aortic strips. After washing, a second addition of the spasmogens gave smaller tonic contractions than the first one. Even more, a third addition of KCl also gave a smaller contraction than the first one, but a complete recovery of the contractile response to noradrenaline was obtained by a third addition of this agonist. Application of cumulative amounts of Ca2+ or Ba2+ (2.4-24 mM) on the residual contraction in response to these agents after nifedipine treatment, but in the absence of the blocker, restored the magnitude of the contractile responses. Addition of cumulative amounts of Mg2+ (2.4-24 mM) did not modify or even relax the contractile responses to KCl and noradrenaline, respectively.

Discrimination by nimodipine, but not by nifedipine, between phasic and tonic contractions of guinea-pig taenia coli induced by K+ depolarization

Effects of Ca2+ channel blockers, such as nifedipine, nimodipine, gallopamil, verapamil, diltiazem, loperamide, Mn2+ and Ni2+, and papaverine, on contractile responses to K+ depolarization were evaluated in longitudinal muscles of taenia coli isolated from guinea-pig. Depolarization with high K+ solution (K+, 40 mM) produced a biphasic (phasic and tonic) contraction, which was inhibited by the above blockers in a concentration-dependent manner. Ratios of IC50 for the phasic contraction to IC50 for the tonic contraction of nimodipine, verapamil, gallopamil, nifedipine, loperamide, diltiazem, papaverine, Ni2+, and Mn2+ were 516.1, 73.7, 22.0, 6.4, 5.3, 4.9, 1.2, 0.7, and 0.1, respectively, indicating that nimodipine suppressed the tonic contraction more effectively than the phasic contraction. In a fluorescence study with fura 2, K+ depolarization elicited an increase in intracellular free Ca2+, [Ca2+]i, which was coupled with the phasic and tonic contraction. The increases in [Ca2+]i coupled with both types of the contraction were abolished by exposure to Ca(2+)-free solution. In addition, the increase of [Ca2+]i coupled with the phasic contraction was abolished by nifedipine, 10(-7) M, but not by nimodipine, 10(-7) M, whereas the increase with the tonic contraction was suppressed by both nifedipine and nimodipine. These findings suggest that the phasic and tonic contractions evoked by K+ depolarization are due to increases in [Ca2+]i via activation of respective nimodipine-resistant and nimodipine-sensitive Ca2+ channels in the longitudinal muscles of the taenia coli. Accordingly, nimodipine, but not nifedipine, appears to be a useful tool for distinguishing between the phasic and tonic contractions.

Quantification of two splicing events in the L-type calcium channel alpha-1 subunit of intestinal smooth muscle and other tissues

cDNA fragments encoding a representative region of the L-type calcium channel alpha-1 subunit of rabbit intestine smooth muscle were amplified by polymerase chain reaction (PCR). The nucleotide sequences of these intestine clones shared a high similarity with aorta, lung and heart calcium channels. However, in the extracellular loop between the third and fourth segments of domain IV and in the transmembrane IVS3 segment itself, we observed primary sequence variations corresponding to alternative splicing phenomenons. Since structural differences of L-type calcium channel alpha-1 subunits could result in functional variations, the respective expression frequency of these isoforms was determined in various tissues and species, and in the embryonic A7r5 cell line. The ontogeny of these splicing events was also examined from tissues of different ages. From this quantitative study, carried out by PCR of reverse-transcribed mRNA, it clearly appears that the observed splicing processes in the IVS3-IVS4 region are not only tissue-dependent but also regulated during development.

The effects of nifedipine on alpha 2-adrenoceptor-mediated contractions in several isolated blood vessels from the rabbit

1. The effects of the dihydropyridine calcium channel blocker, nifedipine, on noradrenaline-induced contractile responses have been examined in several isolated blood vessels from the rabbit, with particular emphasis on responses mediated via postjunctional alpha 2-adrenoceptors. 2. In the isolated renal vein, ear vein, distal saphenous artery, saphenous vein and plantaris vein, 0.1 microM and 1 microM nifedipine reduced responses elicited by 54 mM KCl by more than 70%. The remaining responses were abolished by alpha-adrenoceptor blockade, suggesting the involvement of noradrenaline released from neurones activating a dihydropyridine-resistant mechanism. 3. In the renal vein (alpha 1-), ear vein (predominantly alpha 2-), distal saphenous artery (alpha 1- greater than alpha 2-), saphenous vein and plantaris vein (alpha 2- greater than alpha 1-), 0.01 microM and 0.1 microM nifedipine produced concentration-related reductions in the maximum response to noradrenaline. However, 1 microM nifedipine was no more effective than 0.1 microM nifedipine and the reduction in the maximum varied from 10-25% of the control response. Thus, a sizeable component of the alpha-adrenoceptor-mediated response in all blood vessels is resistant to dihydropyridine calcium channel blockers and this appears to be unrelated to the alpha-adrenoceptor subtype involved. 4. Following irreversible inactivation of alpha 1-adrenoceptors and isolation of functional alpha 2-adrenoceptors in the saphenous vein, plantaris vein and distal saphenous artery (the latter requiring the presence of angiotensin II), the effect of nifedipine on responses to noradrenaline was increased. However, a component of the alpha 2-adrenoceptor response in each preparation was present even after the concentration of nifedipine was increased to 1 microM. 5. In the saphenous vein, a preparation in which it has been demonstrated previously that alpha 2-adrenoceptor-mediated responses are highly dependent upon the presence of extracellular calcium ions, partial depolarization with 20mM KCl failed to increase the inhibitory effect of 0.1 microM nifedipine. This suggests the involvement of dihydropyridine-resistant Ca2+ channels. The possible relationship between these dihydropyridine-resistant Ca2+ channels, alpha-adrenoceptor subtypes and 'receptor-operated' Ca2 + channels is discussed.

Characterization in rat aorta of the binding sites responsible for blockade of noradrenaline-evoked calcium entry by nisoldipine

1. The effectiveness of the calcium antagonist, 1,4-dihydropyridine nisoldipine, as an inhibitor of contraction and 45Ca entry evoked by noradrenaline in rat aorta has been investigated and correlated with binding characteristics in intact artery. 2. Contractions evoked by noradrenaline were concentration-dependently depressed by nisoldipine (0.3-300 nM). About 60% of the response was resistant to inhibition, while KCl-induced contractions could be completely blocked. Noradrenaline-induced contractions were also less sensitive to nisoldipine inhibition than were KCl-induced contractions. 3. Preincubation of the aorta with nisoldipine in high KCl depolarizing solution increased the inhibition of the contraction evoked by a short application of noradrenaline or KCl to a similar extent. 4. The inhibition by nisoldipine of 45Ca influx evoked either by KCl depolarizing solution or by noradrenaline correlated well with the inhibition of the contractile responses. However, while KCl-stimulated 45Ca influx was totally abolished by nisoldipine (300 nM), 38% of the noradrenaline-stimulated 45Ca influx was resistant to inhibition by nisoldipine (300 nM). 5. The study of [3H]-(+)-PN 200-10 ([3H]-(+)-isradipine) binding in intact aorta showed the presence of a homogeneous population of specific binding sites. KD values were dependent on the KCl concentration in the bath while Bmax was unaffected. Binding of [3H]-(+)-isradipine was also increased in tissue exposed to noradrenaline; in the presence of 10(-5) M noradrenaline, binding parameters of [3H]-(+)-isradipine were close to the values obtained in aorta bathed in 20 mM KCl solution. 6. Displacement of [3H]-(+)-isradipine specific binding by nisoldipine was determined in segments of mesenteric artery and of aorta. The potency of nisoldipine was dependent on the incubation conditions applied to the vessel, as follows: KCl (100 mM) depolarizing solution greater than noradrenaline (10(-5) M) = KCl (25 mM) solution greater than physiological solution. The Ki value measured in aorta exposed to noradrenaline (10(-5) M) was close to the IC50 value of nisoldipine on the noradrenaline-evoked contraction. 7. The membrane potential value of rat aorta was estimated by the distribution of [3H]-tetraphenylphosphonium bromide ([3H]-TPP+), [3H]-TPP+ uptake concentration-dependently decreased when the KCl concentration in the bath was increased from 5.9 to 130 mM. Noradrenaline also concentration-dependently decreased [3H]-TPP+ uptake; the maximum effect (1-10 microns noradrenaline) was comparable in amplitude to the effect of 25 mM KCl solution. 8. It is concluded that in rat aorta, noradrenaline activates voltage-operated calcium channels that contain the specific, voltage-sensitive binding sites for calcium antagonistic dihydropyridines. The existence of a fraction of noradrenaline-stimulated '"Ca entry that is resistant to nisoldipine blockade suggests that another Ca2 + entry pathway is also opened by the agonist.