This study compared Na(+)-Ca2+ exchange from the hearts of rainbow trout with that from canines. In several respects, trout cardiac Na(+)-Ca2+ exchange is functionally similar to that from dogs and other mammals. Trout cardiac Na(+)-Ca2+ exchange is stimulated approximately 200% after 30-min incubation with 10 micrograms/ml chymotrypsin at 21 degrees C, similar to mammals. On the other hand, both the temperature and pH dependencies are strikingly different between the trout and canine myocardial Na(+)-Ca2+ exchange. While canine heart Na(+)-Ca2+ exchange exhibits a Q10 of greater than 2 (similar to values observed in other mammals), that from trout is relatively insensitive to temperature with a Q10 of approximately 1.2. The absolute rates of Na(+)-Ca2+ exchange in trout heart are four- to sixfold higher than that in mammals when measured at 7 degrees C. Furthermore, the temperature insensitivity of trout myocardial Na(+)-Ca2+ exchange is retained when the exchanger is reconstituted into an asolectin bilayer, suggesting that this property is intrinsic to the protein and not dependent on species differences in lipid bilayer composition. Trout Na(+)-Ca2+ exchange is not markedly stimulated by alkaline pH, in contrast to mammals, and this characteristic is also maintained after reconstitution. Western blots of trout cardiac sarcolemma run on 7.5% sodium dodecyl sulfate-polyacrylamide gel electrophoresis react with antibodies raised against the canine Na(+)-Ca2+ exchanger with a similar pattern of bands (70, 120, and 160 kDa). Furthermore, a cDNA probe from canine Na(+)-Ca2+ exchanger hybridizes on Northern blots of trout heart mRNA to a 7-kb band, similar to that in mammals. Thus, while important functional differences in Na(+)-Ca2+ exchange exist between trout and mammalian hearts, the molecular basis is not yet known.

Reverse-mode activity of the Na(+)/Ca(2+) exchanger (NCX) has been previously shown to play a prominent role in excitation-contraction coupling in the neonatal rabbit heart, where we have proposed that a restricted subsarcolemmal domain allows a Na(+) current to cause an elevation in the Na(+) concentration sufficiently large to bring Ca(2+) into the myocyte through reverse-mode NCX. In the present study, we tested the hypothesis that there is an overlapping expression and distribution of voltage-gated Na(+) (Na(v)) channel isoforms and the NCX in the neonatal heart. For this purpose, Western blot analysis, immunocytochemistry, confocal microscopy, and image analyses were used. Here, we report the robust expression of skeletal Na(v)1.4 and cardiac Na(v)1.5 in neonatal myocytes. Both isoforms colocalized with the NCX, and Na(v)1.5-NCX colocalization was not statistically different from Na(v)1.4-NCX colocalization in the neonatal group. Western blot analysis also showed that Na(v)1.4 expression decreased by sixfold in the adult (P < 0.01) and Na(v)1.1 expression decreased by ninefold (P < 0.01), whereas Na(v)1.5 expression did not change. Although Na(v)1.4 underwent large changes in expression levels, the Na(v)1.4-NCX colocalization relationship did not change with age. In contrast, Na(v)1.5-NCX colocalization decreased ∼50% with development. Distance analysis indicated that the decrease in Na(v)1.5-NCX colocalization occurs due to a statistically significant increase in separation distances between Na(v)1.5 and NCX objects. Taken together, the robust expression of both Na(v)1.4 and Na(v)1.5 isoforms and their colocalization with the NCX in the neonatal heart provides structural support for Na(+) current-induced Ca(2+) entry through reverse-mode NCX. In contrast, this mechanism is likely less efficient in the adult heart because the expression of Na(v)1.4 and NCX is lower and the separation distance between Na(v)1.5 and NCX is larger.

The high activity of the cardiac Na+-Ca2+ exchanger has led to the suggestion that it plays an important role in the regulation of myocardial contractility. We have proposed that exercise training increases stroke volume as a consequence of an enhanced contractility caused by an adaptation in Ca2+ transport across the cardiac plasma membrane (sarcolemma). The present study examined the possibility that the Na+-Ca2+ exchanger in heart muscle is modified in response to training. Sprague-Dawley rats (female, n = 72) were randomly divided into exercise-trained (T) and sedentary control (C) groups. As a result of the 11-wk treadmill-training paradigm, group T had a 7.6% higher (P less than 0.005) heart-to-body weight ratio and a 36% increase (P less than 0.01) in gastrocnemius mitochondrial enzyme activity. Na+-Ca2+ exchange was studied in highly purified sarcolemmal vesicles using rapid-quenching techniques. The absolute initial rate of uptake was significantly higher in T vs. C at calcium concentrations [( Ca2+]) ranging from 10 to 80 microM. This increased uptake appears to be due solely to the fact that the apparent Km of the myocardial Na+-Ca2+ exchanger for Ca2+ was significantly lower in T vs. C (15.7 +/- 1.1 vs. 36.1 +/- 2.6 microM), since the maximum velocity was unchanged. The observed increase in the affinity of the exchanger for Ca2+ is not attributable to group differences in vesicular purity, cross-contamination, or passive Ca2+ efflux. This observation is consistent with observed alterations in sarcolemmal composition in response to exercise training. We propose that the modification of the Na+-Ca2+ exchanger may play an important role in the adaptation of the heart to exercise.

Abnormalities in cardiac function have been extensively documented in experimental and clinical diabetes. These aberrations are well known to be exaggerated when hypertension and diabetes co-exist. The objective of the present study was to examine whether alterations in the activity of the myocardial Na + -Ca 2+ exchanger (NCX) can account for the deleterious effects of diabetes and (or) hypertension on the heart. To this aim, the following experimental groups were studied: (i) control; (ii) diabetic; (iii) hypertensive; and (iv) hypertensive-diabetic. Wistar rats served as the control group (C) while Wistar rats injected with streptozotocin (STZ, 55 mg/kg) served as the diabetic (D) group. Spontaneously hypertensive (SH) rats were used as the hypertensive group (H) while SH rats injected with STZ served as the hypertensive-diabetic (HD) group. Sarcolemma was isolated from the ventricles of the C, D, H, and HD groups and NCX activity was examined using rapid quenching techniques to study initial rates over a [Ca 2+ ] o range of 10-160 µM. The V max of NCX was lower in the D group when compared with the C group (D, 2.96 ± 0.26 vs. C, 4.0 ± 0.46 nmol·mgprot -1 ·s -1 , P < 0.05), however combined diabetes and hypertension (HD) did not affect the V max of NCX activity (HD, 3.84 ± 0.88 vs. H, 3.59 ± 0.24 nmol·mgprot -1 ·s -1 , P > 0.05). However, analysis of the K m values for Ca 2+ indicated that both the D and HD groups exhibited a significantly lower K m when compared with their respective control groups (D, 42 ± 4 vs. C, 56 ± 4 µM, P < 0.05; HD, 33 ± 7 vs. H, 51 ± 8 µM, P < 0.05). Immunoblotting using polyclonal antibodies (against canine cardiac NCX) exhibited the typical banding of 160, 120, and 70 kDa. The 120 kDa band is believed to represent the native exchanger with its post-translational modifications. Examination of the blots revealed a lower intensity of the 120 kDa band in the D group when compared with the C group, however, no significant difference in the HD group was observed. We speculate that the lower V max in the D group may be due to a reduced concentration of exchanger protein in the membrane. The absence of this defect in the HD group may be a result of compensatory mechanisms to the overall hemodynamic overload, however, this remains to be determined. The increased affinity for Ca 2+ in both the D and HD groups (determined by the lower K m values) is an interesting finding and may be due to changes in sarcolemmal lipid bilayer composition secondary to diabetes-induced hyperlipidemia.Résumé : Les anomalies de la fonction cardiaque au cours du diabète clinique et expérimental ont été largement documentées. On sait que ces aberrations sont amplifiées lorsque diabète et hypertension coexistent. La présente étude a eu pour but d'examiner si les altérations de l'activité de l'échangeur Na + -Ca 2+ (NCX) myocardique peuvent expliquer les effets nocifs du diabète et/ou de l'hypertension sur le coeur. Les groupes expérimentaux suivants ont été examinés : (i) témoin, (ii) diabétique, (iii) hypertendu, e...