Molecular mechanism of use-dependent calcium channel block by phenylalkylamines: role of inactivation

The role of channel inactivation in the molecular mechanism of calcium (Ca2+) channel block by phenylalkylamines (PAA) was analyzed by designing mutant Ca2+ channels that carry the high affinity determinants of the PAA receptor site [Hockerman, G. H., Johnson, B. D., Scheuer, T., and Catterall, W. A. (1995) J. Biol. Chem. 270, 22119-22122] but inactivate at different rates. Use-dependent block by PAAs was studied after expressing the mutant Ca2+ channels in Xenopus oocytes. Substitution of single putative pore-orientated amino acids in segment IIIS6 by alanine (F-1499-A, F-1500-A, F-1510-A, I-1514-A, and F-1515-A) gradually slowed channel inactivation and simultaneously reduced inhibition of barium currents (I(Ba)) by (-)D600 upon depolarization by 100 ms steps at 0.1 Hz. This apparent reduction in drug sensitivity was only evident if test pulses were applied at a low frequency of 0.1 Hz and almost disappeared at the frequency of 1 Hz. (-)D600 slowed I(Ba) recovery after maintained membrane depolarization (1-3 sec) to a comparable extent in all channel constructs. A drug-induced delay in the onset of I(Ba) recovery from inactivation suggests that PAAs promote the transition to a deep inactivated channel conformation. These findings indicate that apparent PAA sensitivity of Ca2+ channels is not only defined by drug interaction with its receptor site but also crucially dependent on intrinsic gating properties of the channel molecule. A molecular model for PAA-Ca2+ channel interaction that accounts for the relationship between drug induced inactivation and channel block by PAA is proposed.

Transfer of high sensitivity for benzothiazepines from L-type to class A (BI) calcium channels

To investigate the molecular basis of the calcium channel block by diltiazem, we transferred amino acids of the highly sensitive and stereoselective L-type (alpha1S or alpha1C) to a weakly sensitive, nonstereoselective class A (alpha1A) calcium channel. Transfer of three amino acids of transmembrane segment IVS6 of L-type alpha1 into the alpha1A subunit (I1804Y, S1808A, and M1811I) was sufficient to support a use-dependent block by diltiazem and by the phenylalkylamine (-)-gallopamil after expression in Xenopus oocytes. An additional mutation F1805M increased the sensitivity for (-)-gallopamil but not for diltiazem. Our data suggest that the receptor domains for diltiazem and gallopamil have common but not identical molecular determinants in transmembrane segment IVS6. These mutations also identified single amino acid residues in segment IVS6 that are important for class A channel inactivation.

Sequence differences between alpha1C and alpha1S Ca2+ channel subunits reveal structural determinants of a guarded and modulated benzothiazepine receptor

The molecular basis of the Ca2+ channel block by (+)-cis-diltiazem was studied in class A/L-type chimeras and mutant alpha1C-a Ca2+ channels. Chimeras consisted of either rabbit heart (alpha1C-a) or carp skeletal muscle (alpha1S) sequence in transmembrane segments IIIS6, IVS6, and adjacent S5-S6 linkers. Only chimeras containing sequences from alpha1C-a were efficiently blocked by (+)-cis-diltiazem, whereas the phenylalkylamine (-)-gallopamil efficiently blocked both constructs. Carp skeletal muscle and rabbit heart Ca2+ channel alpha1 subunits differ with respect to two nonconserved amino acids in segments IVS6. Transfer of a single leucine (Leu1383, located at the extracellular mouth of the pore) from IVS6 alpha1C-a to IVS6 of alpha1S significantly increased the (+)-cis-diltiazem sensitivity of the corresponding mutant L1383I. An analysis of the role of the two heterologous amino acids in a L-type alpha1 subunit revealed that corresponding amino acids in position 1487 (outer channel mouth) determine recovery of resting Ca2+ channels from block by (+)-cis-diltiazem. The second heterologous amino acid in position 1504 of segment IVS6 (inner channel mouth) was identified as crucial inactivation determinant of L-type Ca2+ channels. This residue simultaneously modulates drug binding during membrane depolarization. Our study provides the first evidence for a guarded and modulated benzothiazepine receptor on L-type channels.

Mechanism of voltage- and use-dependent block of class A Ca2+ channels by mibefradil

1. The action of mibefradil was studied on wild type class A calcium (Ca2+) channels and various class A/L-type channel chimaeras expressed in Xenopus oocytes. The mechanism of Ca2+ channel block by mibefradil was evaluated with two microelectrode voltage clamp. 2. Resting-state dependent block (or initial block) of barium currents (IBa) through class A Ca2+ channels was concentration dependent with an IC50 value of 208+/-23 microM. 3. Mibefradil (50 microM) did not significantly affect the midpoint voltage of the steady-state inactivation curve suggesting that inactivation does not promote Ca2+ channel block. Chimaeric class A/L-type Ca2+ channels inactivating with faster or slower kinetics than wild type class A channels were equally well inhibited by mibefradil as wild type class A channels. 4. Frequent Ca2+ channel activation facilitated IBa inhibition by mibefradil (use-dependent block). Recovery from use-dependent block was voltage-dependent, being slower at depolarized membrane potentials (tau = 75+/-15 s at -70 mV, (n=6) vs tau = 20+/-2 s at -100 mV, (n=6), P<0.05). 5. We suggest that use-dependent block of class A Ca2+ channels by mibefradil occurs because of slow recovery from open channel block (SROB) and not because of drug binding to inactivated channels. 6. Voltage-dependent slow recovery from open state-dependent block provides a molecular basis for understanding the cardiovascular profile of mibefradil such as selectivity for vasculature and relative lack of negative inotropic effects.

Leptin, leptin soluble receptor and coronary atherosclerosis

BACKGROUND

The adipose tissue-related hormone leptin plays an important role in the regulation of body weight. The associations of leptin and leptin soluble receptor (sOb-R) with coronary artery disease (CAD) are not clear.

DESIGN

We measured leptin and sOb-R in 543 consecutive patients (379 men, 164 women) referred for coronary angiography for the evaluation of CAD. Coronary artery stenoses with lumen narrowing > or = 50% were considered significant.

RESULTS

Serum leptin correlated significantly with body mass index (r(s) = 0.443), with insulin resistance as assessed by the homeostasis model for the assessment of insulin resistance (r(s) = 0.339), with serum triglycerides (r(s) = 0.181), with systolic as well as diastolic blood pressure (r(s) = 0.170 and r(s) = 0.133, respectively) and, inversely, with sOb-R (r(s) = -0.346; P < 0.01 for all correlations). Coronary angiography revealed significant coronary artery stenoses in 331 (61%) of our patients. Serum leptin was significantly lower in patients with significant coronary artery stenoses than in patients without such lesions (8.5 +/- 7.8 vs. 13.2 +/- 12.2 ng mL(-1); P < 0.001). Multivariate logistic regression analysis proved serum leptin inversely and independently associated with the presence of significant coronary artery stenoses (standardized adjusted odds ratio 0.746, 95% confidence interval 0.566-0.983, P = 0.038). In contrast to serum concentrations of leptin, serum concentrations of sOb-R did not significantly differ between patients with significant stenoses and those without such lesions (22.4 +/- 8.3 vs. 23.1 +/- 12.1 ng mL(-1); P = 0.655).

CONCLUSIONS

Serum leptin but not sOb-R is significantly lower in patients with angiographically determined CAD. Despite its association with cardiovascular risk factors, leptin should not be simply regarded as a promoter of atherosclerosis.

Calcium channel block by (-)devapamil is affected by the sequence environment and composition of the phenylalkylamine receptor site

The pore-forming alpha 1 subunit of L-type calcium (Ca2+) channels is the molecular target of Ca2+ channel blockers such as phenylalkylamines (PAAs). Association and dissociation rates of (-)devapamil were compared for a highly PAA-sensitive L-type Ca2+ channel chimera (Lh) and various class A Ca2+ channel mutants. These mutants carry the high-affinity determinants of the PAA receptor site in a class A sequence environment. Apparent drug association and dissociation rate constants were significantly affected by the sequence environment (class A or L-type) of the PAA receptor site. Single point mutations affecting the high-affinity determinants in segments IVS6 of the PAA receptor site, introduced into a class A environment, reduced the apparent drug association rates. Mutation I1811M in transmembrane segment IVS6 (mutant AL25/-I) had the highest impact and decreased the apparent association rate for (-)devapamil by approximately 30-fold, suggesting that this pore-lining isoleucine in transmembrane segment IVS6 plays a key role in the formation of the PAA receptor site. In contrast, apparent drug dissociation rates of Ca2+ channels in the resting state were almost unaffected by point mutations of the PAA receptor site.

Relationship between various measures of bone mineral density and vertebral fractures in cardiac transplant recipients

BACKGROUND

The incidence rates of osteoporosis and fractures are increased after cardiac transplantation.

METHODS

We performed a cross-sectional analysis of cardiac transplant recipients in a late post-transplantation period (4.4 [2.5] years after cardiac transplantation, n = 27). We measured bone mineral density (BMD) by DXA at the hip and lumbar spine and by quantitative ultrasound (QUS) at the calcaneus. Vertebral fracture (vfx) prevalence was analysed by anterior-posterior and lateral radiographs of the thoracic and lumbar spine.

RESULTS

Overall, vfx were present in 13 of 27 patients (48.2%, n = 51 vfx). Vfx were observed in 1 out of 5 patients with normal DXA results, 7 out of 14 osteopenic and 5 out of 8 osteoporotic cardiac transplant recipients. BMD at the femoral neck and more prominently at Ward's triangle were significantly lower in vfx patients compared to patients without vfx, with adjusted mean values (95% CI) of 0.804 [0.750-0.859] vs. 0.915 [0.860-0.969] g/cm2 and 0.573 [0.501-0.646] vs. 0.766 [0.697-0.836] g/cm2, respectively.

CONCLUSIONS

These findings suggest an association between DXA measurements of the hip with vertebral fractures in a late post-transplantation period and thus extend knowledge from previous reports on cardiac transplant recipients studied earlier after CTX. In particular, our data pinpoint a potentially interesting role for BMD at Ward's triangle.

Relationship between various measures of bone mineral density and vertebral fractures in cardiac transplant recipients

BACKGROUND

The incidence rates of osteoporosis and fractures are increased after cardiac transplantation.

METHODS

We performed a cross-sectional analysis of cardiac transplant recipients in a late post-transplantation period (4.4 [2.5] years after cardiac transplantation, n = 27). We measured bone mineral density (BMD) by DXA at the hip and lumbar spine and by quantitative ultrasound (QUS) at the calcaneus. Vertebral fracture (vfx) prevalence was analysed by anterior-posterior and lateral radiographs of the thoracic and lumbar spine.

RESULTS

Overall, vfx were present in 13 of 27 patients (48.2%, n = 51 vfx). Vfx were observed in 1 out of 5 patients with normal DXA results, 7 out of 14 osteopenic and 5 out of 8 osteoporotic cardiac transplant recipients. BMD at the femoral neck and more prominently at Ward's triangle were significantly lower in vfx patients compared to patients without vfx, with adjusted mean values (95% CI) of 0.804 [0.750-0.859] vs. 0.915 [0.860-0.969] g/cm2 and 0.573 [0.501-0.646] vs. 0.766 [0.697-0.836] g/cm2, respectively.

CONCLUSIONS

These findings suggest an association between DXA measurements of the hip with vertebral fractures in a late post-transplantation period and thus extend knowledge from previous reports on cardiac transplant recipients studied earlier after CTX. In particular, our data pinpoint a potentially interesting role for BMD at Ward's triangle.