It has been reported earlier that the slow (C-type) inactivated conformation in Kv channels is stabilized by a multipoint hydrogen-bond network behind the selectivity filter. Furthermore, MD simulations revealed that structural water molecules are also involved in the formation of this network locking the selectivity filter in its inactive conformation. We found that the application of an extracellular, but not intracellular, solution based on heavy water (D2O) dramatically slowed entry into the slow inactivated state in Shaker-IR mutants (T449A, T449A/I470A, and T449K/I470C, displaying a wide range of inactivation kinetics), consistent with the proposed effect of the dynamics of structural water molecules on the conformational stability of the selectivity filter. Alternative hypotheses capable of explaining the observed effects of D2O were examined. Increased viscosity of the external solution mimicked by the addition of glycerol had a negligible effect on the rate of inactivation. In addition, the inactivation time constants of K+ currents in the outward and the inward directions in asymmetric solutions were not affected by a H2O/D2O exchange, negating an indirect effect of D2O on the rate of K+ rehydration. The elimination of the nonspecific effects of D2O on our macroscopic current measurements supports the hypothesis that the rate of structural water exchange at the region behind the selectivity filter determines the rate of slow inactivation, as proposed by molecular modeling.

No abstract

Background Comparison of the performance of a conventional, Anger-system, general-purpose SPECT/CT (AnyScan SC FLEX) vs. an Anger-system dedicated cardiac SPECT (CardioC) camera for gated myocardial perfusion imaging. Method Fifty patients were enrolled into the study. Of these, myocardial perfusion scintigraphy with 450 MBq SestaMIBI (Mediradiopharma) was performed after dipyridamole stress testing in 43, and at rest in the remaining 7 patients. ECG-gated SPECT acquisition was undertaken with both cameras, with a scanning time of 16 minutes. Within 45 to 60 minutes of the intravenous administration of the radiopharmaceutical, imaging was done first with the CardioC and then – after 30 minutes on average – with the AnyScan SC FLEX camera. The sensitivity of the systems (uptake of the radiotracer by the heart), severity of perfusion defects (SS), and parameters of left ventricular volume and wall motion (EF, EDV, ESV, SV, and cardiac mass) were determined quantitatively with both cameras. All these indices were then compared and the results were subjected to statistical analysis. Results Sensitivity (the scintillation counts recorded within the ROI cardiac projection) was 17.8% (median) higher with the AnyScan FLEX than with the CardioC camera (p < 0.001). There was no meaningful difference between the two cameras in assessing the severity of the perfusion abnormality (p > 0.1). The EF obtained after attenuation correction was significantly lower than without AC. The volume estimates significantly depended on the camera and reconstruction method (p < 0.001); especially, with the FLEX camera the EDV and SV estimates were significantly lower. Attenuation correction of the FLEX images resulted in the lowest estimated myocardial mass(p = 0.004 against the values obtained with CardioC). Conclusions The results support our initial assumption that when used formyocardial perfusion imaging, the performance of general-purpose, digital, hybrid SPECT/CT cameras and dedicated cardiac scanners (both with parallel-hole collimators) is similar in the case of conventional Anger-system devices.