Sarcolemmal ATPase activities of the rat heart ventricle--dependence on age and sodium ion

Na-K-ATPase activity of the rat heart was similar throughout the postnatal growth when measured from crude unpurified fraction. Instead in the cardiac sarcolemmal fraction, isolated by hypotonic shock LiBr-treatment method, the activity was over two times higher in 10-day old neonates than in adult rats. The conflicting results are partly explained by different effects of the isolation procedure on neonatal and adult tissues. Na concentration for half-maximal activity of the Na-K-ATPase was similar in neonates (7.0 mM) and adults (6.4 mM). Ca-ATPase activity was not affected by Na concentration (2-100 mM) in the two age-groups studied.     

Postnatal development of beta-adrenergic response in ventricular muscle of the rat heart

In adult mammalian, heart responses to beta- and alpha-adrenergic stimulation are different: the beta-type effect exhibits a larger increase of relaxation than of contraction, while the alpha-stimulation has no selective influence on relaxation. The present results show that the effect of isoprenaline (ISO) on the neonatal rat heart during the 1st postnatal week is not a typical beta-effect in that the relaxant influence of beta-stimulation is lacking. During the 2nd and 3rd postnatal weeks the typical beta-response, with improved relaxation, gradually appears. The absence of the typical beta-effect is not caused by the lack of beta-receptors or cAMP-dependent phosphorylation reactions because in other respects, the positive inotropic effect of ISO is well developed at the moment of birth. In addition to these qualitative changes, also prominent quantitative changes occurred in the ISO response. The dose-response curves were shifted to the right with advancing age, suggesting reduced beta-agonist potency of the maturing tissue. The developed tension (Tmax) abruptly increased between the 12th and 17th postnatal days and then steeply declined during the next 2 weeks. Changes in Tmax correlated fairly well with the general ability of the tissue to generate extra force, as expressed by rest-dependent potentiation of twitch. However, during the 2nd postnatal week cardiac tissue seemed to be subsensitive to ISO, since all contractile parameters except T'max were depressed. The results suggest that the postnatal changes in beta-response are primarily determined by alternations in the electromechanical coupling process of the developing tissue, and less by the proper adrenergic mechanisms.     

Characterization of contractions in enzymatically isolated rat ventricular myocytes: effects of ouabain and rubidium

The role of sarcolemma and especially sodium pump activity in the control of phasic contractile activity of Ca2+ tolerant myocytes was studied using ouabain and rubidium as sodium pump inhibitors. Initially, ouabain increased both the amplitude of shortening and the frequency of phasic contractions. Later, the amplitude began to decline whereas the frequency of beating continued to rise, often terminating in a steady contracture of the myocyte. Rubidium caused a rapid rise of beating frequency, which reached its full effect within 1-5 min and remained steady after that. The stimulation of contraction frequency and the inhibition of Na+-K+ ATPase were correlated in the case of ouabain but not in the case of rubidium. The results suggest that the stimulation of phasic contractions may be caused by increased uptake of cellular calcium through Na+-Ca+ exchange as a consequence of sodium pump inhibition and (or) depolarization of the sarcolemma by ouabain and rubidium.     

Inhibition of the cardiac inward rectifier potassium currents by KB-R7943

KB-R7943 (2-[2-[4-(4-nitrobenzyloxy)phenyl]ethyl]isothiourea) was developed as a specific inhibitor of the sarcolemmal sodium–calcium exchanger (NCX) with potential experimental and therapeutic use. However, KB-R7943 is shown to be a potent blocker of several ion currents including inward and delayed rectifier K⁺ currents of cardiomyocytes. To further characterize KB-R7943 as a blocker of the cardiac inward rectifiers we compared KB-R7943 sensitivity of the background inward rectifier (I_K1) and the carbacholine-induced inward rectifier (I_KACh) currents in mammalian (Rattus norvegicus; rat) and fish (Carassius carassius; crucian carp) cardiac myocytes. The basal I_K1 of ventricular myocytes was blocked with apparent IC50-values of 4.6 × 10^− 6 M and 3.5 × 10^− 6 M for rat and fish, respectively. I_KACh was almost an order of magnitude more sensitive to KB-R7943 than I_K1 with IC₅₀-values of 6.2 × 10^− 7 M for rat and 2.5 × 10^− 7 M for fish. The fish cardiac NCX current was half-maximally blocked at the concentration of 1.9–3 × 10^− 6 M in both forward and reversed mode of operation. Thus, the sensitivity of three cardiac currents to KB-R7943 block increases in the order I_K1 ~ I_NCX < I_KACh. Therefore, the ability of KB-R7943 to block inward rectifier potassium currents, in particular I_KACh, should be taken into account when interpreting the data with this inhibitor from in vivo and in vitro experiments in both mammalian and fish models.     

Effects of chronic hypoxia on inward rectifier K(+) current ( I(K1)) in ventricular myocytes of crucian carp (Carassius carassius) heart

Some ectothermic vertebrates show unusually good tolerance to oxygen shortage and it is therefore assumed that they might, as a defense mechanism, decrease number or activity of ion channels in order to reduce membrane leakage and thereby ATP-dependent ion pumping in hypoxia. Although several studies have provided indirect evidence in favor of this channel arrest hypothesis, only few experiments have examined activity of ion channels directly from animals exposed to chronic hypoxia or anoxia in vivo. Here we compare the inwardly rectifying K⁺ current (I_K1), a major leak and repolarizing K⁺ pathway of the heart, in cardiac myocytes of normoxic and hypoxic crucian carp, using the whole-cell and cell-attached single-channel patch-clamp methods. Whole-cell conductance of I_K1 was 0.5 ± 0.04 nS/pF in normoxic fish and did not change during the 4 weeks hypoxic (O₂ < 0.4 mg/l; 2.68 mmHg) period, meanwhile the activity of Na⁺/K⁺ATPase decreased 33%. Single-channel conductance of the I_K1 was 20.5 ± 0.8 pS in control fish and 21.4 ± 1.1pS in hypoxic fish, and the open probability of the channel was 0.80 ± 0.03 and 0.74 ± 0.04 (P > 0.05) in control and hypoxic fish, respectively. Open and closed times also had identical distributions in normoxic and hypoxic animals. These results suggest that the density and activity of the inward rectifier K⁺ channel is not modified by chronic hypoxia in ventricular myocytes of the crucian carp heart. It is concluded that instead of channel arrest, the hypoxic fish cardiac myocytes obtain energy savings through action potential arrest due to hypoxic bradycardia.     

Cold acclimation increases basal heart rate but decreases its thermal tolerance in rainbow trout (Oncorhynchus mykiss)

Rainbow trout (Oncorhynchus mykiss, Walbaum) were acclimated to 4 degrees C and 17 degrees C for more than 4 weeks and heart rate was determined in the absence and presence of adrenaline to see how thermal adaptation influences basal heart rate and its beta-adrenergic control in a eurythermal fish species. The basal heart rate in vitro was higher in cold-acclimated than warm-acclimated rainbow trout at temperatures below 17 degrees C. On the other hand, adaptation to cold decreased thermal tolerance of heart rate so that the maximal heart rates were achieved at 17 degrees C (75 +/- 4 bpm) and 24 degrees C (88 +/- 2 bpm) in cold-acclimated and warm-acclimated trout, respectively. Beta-adrenergic response of the heart was enhanced by cold-adaptation, since adrenaline (100 nmol l(-1)) caused stronger stimulation of heart rate in cold-acclimated (29 +/- 14%) than in warm-acclimated fish (10 +/- 1%; P = 0.03). Furthermore, adrenaline strongly opposed the temperature-dependent deterioration of force production in cold-acclimated trout but not in warm-acclimated trout. The results indicate that adaptation to cold increases basal heart rate but decreases its thermal tolerance in rainbow trout. Cold acclimation up-regulates the beta-adrenergic system, and beta-adrenoceptor activation seems to provide cardioprotection against high temperatures in the cold-adapted rainbow trout.     

Fiber capillarization relative to mitochondrial volume in diaphragm of shrew.

The objective was to examine fiber capillarization in relation to fiber mitochondrial volume in the highly aerobic diaphragm of the shrew, the smallest mammal. The diaphragms of four common shrews [Sorex araneus; body mass, 8.2 +/- 1.3 (SE) g] and four lesser shrews (Sorex minutus, 2.6 +/- 0.1 g) were perfusion fixed in situ, processed for electron microscopy, and analyzed by morphometry. Capillary length per fiber volume was extremely high, at values of 8,008 +/- 1,054 and 12,332 +/- 625 mm(-2) in S. araneus and S. minutus, respectively (P = 0.012), with no difference in capillary geometry between the two species. Fiber mitochondrial volume density was 28.5 +/- 2.3% (S. araneus) and 36.5 +/- 1.4% (S. minutus; P = 0.025), yielding capillary length per milliliter mitochondria values (S. araneus, 27.8 +/- 1.5 km; S. minutus, 33.9 +/- 2.2 km; P = 0.06) as high as in the flight muscle of the hummingbird and small bats. The size of the capillary-fiber interface (i.e., capillary surface per fiber surface ratio) per fiber mitochondrial volume in shrew diaphragm was also as high as in bird and bat flight muscles, and it was about two times greater than in rat hindlimb muscle. Thus, whereas fiber capillary and mitochondrial volume densities decreased with increased body mass in S. araneus compared with S. minutus Soricinae shrews, fiber capillarization per milliliter mitochondria in both species was much higher than previously reported for shrew diaphragm, and it matched that of the intensely aerobic flight muscles of birds and mammals.