Effects of dbcAMP and theophylline on rat adrenal medulla grown in tissue culture.

Explants of rat adrenal medulla were grown in tissue culture. The effects of various doses of dbcAMP ranging from 0.001 mM up to 1 mM and equimolar amounts of theophylline were recorded by phase contrast optics and catecholamine histochemistry (glyoxylic acid method) over six days. There was a dose-dependent inhibition of the normally occurring outgrowth of Schwann cells, "chromaffin" cells and axons from the explants. Maintenance of glyoxylic acid-induced fluorescence in "chromaffin" cells was dose-dependent, too. Since theophylline is known to enhance intracellular levels of cAMP only, these effects are probably due to the action of cAMP. cAMP obviously maintains the degree of differentiation of chromaffin cells. Thus it could be argued that a certain degree of dedifferentiation is a prerequisite for the formation of axons from these cells.     

Effect of “Chemical” Hypoxia on the Potassium Conductance of the Membrane of Pheochromocytoma Cells

We studied the effect of “chemical” (induced by the action of sodium thiosulfate, STS) hypoxia on the potassium conductance of the membrane of pheochromocytoma cells. Application of 1 to 10 mM STS decreased in a dose-dependent manner the amplitude of integral potassium current without changes in the voltage dependence of its activation. The concentration dependence of the action of STS on the amplitude of potassium current was estimated using the Boltzmann equation. The value of concentration for 50% inhibition was 2.7 ± 0.2 mM, while the slope coefficient was 0.9 ± 0.2 mM⁻¹. In the presence of 10 mM STS, the decrease in the amplitude of potassium current reached, on average, 55%. Therefore, “chemical” hypoxia influences rather significantly the potassium conductance of the membrane of pheochromocytoma PC12 cells.     

Collagen prolyl hydroxylation in WI-38 fibroblast cultures: Action of hydralazine

Summary The action of hydralazine on collagen prolyl hydroxylation was studied in a cell culture system using WI-38 fibroblasts. The prolyl hydroxylation level was determined by a method involving the digestion of collagen by bacterial collagenase and the examination of specific peptides. The presence of low concentrations of hydralazine (0.2 mM) in both “young” and “old” fibroblast cultures strongly inhibited collagen prolyl hydroxylation. The degree of inhibition was greater in serum-deficient cultures. No significant improvement in the degree of hydroxylation was observed by increasing either ascorbate or iron levels in the hydralazine-containing cultures in which hydroxylation was inhibited. Some of the reported side effects of hydralazine seen in patients might be related to its inhibitory effects on mixed function oxidative (MFO) hydroxylation systems. While the ascorbate dependence of the prolyl hydroxylase system of WI-38 decreased with the “age” of the culture, hydralazine inhibition of hydroxylation was dramatic with cultures of all “ages”. This work was supported by NIH grants nos. AM15671, AM1675 and HD07376, and fellowship no. HD01998.     

Effect of sodium butyrate on mammalian cells in culture: A review

Summary Sodium butyrate produces reversible changes in morphology, growth rate, and enzyme activities of several mammalian cell types in culture. Some of these changes are similar to those produced by agents which increase the intracellular level of adenosine 3′,5′-cyclic monophosphate (cAMP) or by analogs of cAMP. Sodium butyrate increases the intracellular level of cAMP by about two fold in neuroblastoma cells; therefore, some of the effects of sodium butyrate on these cells may in part be mediated by cAMP. Sodium butyrate appears to have properties of a good chemotherapeutic agent for neuroblastoma tumors because the treatment of neuroblastoma cells in culture causes cell death and “differentiation”; however, it is either innocuous or produces reversible morphological and biochemical alterations in other cell types.     

Subpopulation of nestin positive glial precursor cells occur in primary adult human brain cultures

Glial fibrillary acidic protein (GFAP) is an intermediate filament protein considered to be the best astroglial marker. However, the predominant cell population in adult human brain tissue cultures does not express GFAP; these cells have been termed “glia-like” cells. The basic question about histological origin of adult human brain cultures remains unanswered. Some authors showed that “glia-like” cells in adult human brain cultures might be of non-glial origin. We examined primary explant tissue cultures derived from 70 adult human brain biopsies. Within first 5–10 days approximately 5–10% of the small explants became attached. Outgrowing cells were mostly flat cells. These cells formed confluent layer over 3–6 weeks in culture. At confluence the cultures contained 2–5% of microglial cells, 0.1% GFAP-positive astrocytes, less than 0.01% oligodendrocytes and 95–98% GFAP-negative “glia-like” cells. This population of flat “glia-like” cells was positively stained for vimentin, fibronectin, and 20–30% of these cells stained for nestin. Our findings revealed that 1 mM dibutyryl-cAMP addition, in serum free conditions, induced a reversible stellation in 5-10% of the flat “glia-like” cells but did not induce the expression of GFAP or nestin in morphologically changed stellate cells. These results demonstrate that “glia-like” cells in primary adult human brain cultures constitute heterogeneous cell populations albeit with similar morphological features. Two distinct subpopulations have been shown: (i) the one immunostained for nestin; and (ii) the other reactive for dibutyryl-cAMP treatment.     

Taurine reduces caspase-8 and caspase-9 expression induced by ischemia in the mouse hypothalamic nuclei

Summary. Taurine is a sulphur-containing amino acid abundant in the nervous system. It protects cells from ischemia-induced apoptosis, but the mechanism underlying this is not well established. The aim of our study was to explore the effects of taurine on two main pathways of apoptosis induced by ischemia: receptor-mediated and mitochondrial cell death. Brain slices containing the supraoptic (SON) and paraventricular (PVN) nuclei of the hypothalamus were incubated in vitro under control and simulated ischemic (oxygen-glucose deprivation for 30 min) conditions in the absence and presence of 20 mM taurine. Brain slices were harvested after the 180-min “postischemic” period and fixed in 4% paraformaldehyde. To estimate apoptosis, immunostaining was done for caspase-8 and caspase-9 in paraffin-embedded sections. Immunoreactive caspase-8 and caspase-9 cells were observed in SON and PVN in all experimental groups, but in the “ischemic” group the expression of caspase-8 and caspase-9 and the number of immunoreactive cells was significantly increased in both hypothalamic nuclei. Addition of taurine (20 mM) to the incubation medium induced a marked decrease in caspase-8 and caspase-9 immunoreactivity after ischemia in SON and PVN when compared with the taurine-untreated “ischemic” group. Taurine reduces ischemia-induced caspase-8 and caspase-9 expression, the key inductors of apoptosis in SON and PVN. Authors’ address: Dr. Andrey Taranukhin, Tampere Brain Research Center, Medical School, University of Tampere, FI-33014 Finland     

Microheological properties of different erythrocyte populations in persons with elevated arterial pressure and in physically active persons

Microrheological properties (aggregation and deformability) of erythrocytes separated by centrifugation at 30000×g density gradient into “young” and “old” (the upper and lower fractions, respectively) were studied. The erythrocytes were taken from physically active persons (PAP) and from those with elevated arterial pressure (EAP). A significant difference in microrheological properties of the “young” and “old” erythrocytes was found. The aggregation degree of “old” cells was nearly twice that of “young” cells. The deformability of the erythrocyte subpopulations was significantly different, though the difference was not so pronounced as in the case of aggregation. The aggregation of “young” and “old” erythrocytes in the PAP group was the least (28% lower than in the control). Note, that “old” erythrocytes in the PAP group had better microrheological properties than in the other groups. All erythrocyte populations in the EAP group were characterized by higher aggregation, decreased deformability, and decreased capacity for oxygen transport.     

Modulation of cyclic AMP in purified rat mast cells. I. Responses to pharmacologic, metabolic, and physical stimuli.

The cyclic adenosine 3', 5'-monophosphate (cAMP) content of isolated unstimulated mast cells and the changes induced by a variety of pharmacologic, metabolic, and physical stimuli were studied. A modified bovine serum albumin density gradient purification method consistently provided mast cell preparations which were 95% or more pure, without apparent damage, and a 73% recovery of the mast cells applied to the gradients. The measured cAMP in unstimulated mast cells was high, a mean of 16 picomoles per million cells. Moderate agitation or contact with glass increased cAMP content about 2-fold. When calcium was omitted from the medium mast cell cAMP was markedly elevated; incremental increases in added calcium ion concentration from 1 muM to 1 mM caused a linear decrease in cAMP content. Theophylline (3 to 20 mM) caused a dose-related increase in mast cell cAMP content, approximately 2-fold at 20 mM theophylline. Epinephrine (0.01 to 1 mM) caused a modest, dose-related increase in cAMP. In the presence of theophylline, epinephrine increased cAMP levels equal to or greater than the sum of the effects of the agents used individually. The increase in cAMP induced by epinephrine was completely inhibited by 100 muM propranolol and partially inhibited by 10 muM propranolol, thus suggesting that a beta adrenergic receptor is involved. Prostaglandin E1 (PGE1) and histamine (in the presence of theophylline) also raised cAMP. Carbamylcholine (1 nM) lowered cAMP 38%; Atropine (0.1 mM) inhibited the decrease in cAMP induced by 1 nM carbamylcholine by 83% indicating that a muscarinic receptor is involved. In these homogeneous single cell suspensions, therefore, cholinergic and beta adrenergic agents have opposing effects on cAMP levels. Diazoxide (10 muM) and adenine (1 muM) caused 37 and 32% decreases in cAMP, respectively. The availability of highly purified mast cells and the identification of agents which either decrease or increase cAMP content allows a direct examination of the role of cAMP in histamine release.     

Ca<Superscript>2+</Superscript> release from intracellular stores of pig oocytes at different stages of growth

Ca²⁺ release from intracellular stores of pig oocytes was investigated using the Ca²⁺-sensitive fluorescent dye chlorotetracycline. Oocytes were divided into growing ones and those that completed their growth using brilliant cresyl blue (BCB) staining. The stained oocytes (BCB “+”) were determined as the ones that completed their growth, while the stainless ones (BCB “−”) were determined as those in the final stages of growth. In the BCB “+” and BCB “−” oocytes, prolactin, theophylline, GTP, and GDP cause Ca²⁺ to exit intracellular stores. In the oocytes that completed their growth, joint action of prolactin and GTP activates additional release of Ca²⁺, in which protein kinase C takes part. In growing oocytes, joint action of prolactin and GTP does not lead to additional release of Ca2⁺. Joint action of theophylline and GDP in growing oocytes and oocytes that completed the growth stage promotes additional Ca²⁺ exit from intracellular stores. This exit is regulated by protein kinase A. The obtained data show that there various routes of Ca²⁺ release from intracellular stores in growing and grown pig oocytes.     

Use of microsatellite DNA and otolith Sr:Ca ratios to infer genetic relationships and migration history of four morphotypes of <Emphasis Type="Italic">Rhinogobius</Emphasis> sp. OR

The genetic diversity and relationship among four morphotypes of Rhinogobius sp. OR, Gobiidae (“Tōshoku,” “Shinjiko,” “Gi-tōshoku,” and “Shimahire”) were investigated with seven microsatellite DNA loci, and amphidromy of these morphotypes was verified by strontium (Sr) and calcium (Ca) deposition in the otolith. Samples of “Tōshoku,” “Shinjiko,” “Gi-tōshoku,” and “Shimahire” were collected from, respectively, three, three, two, and four locations in Japan. Microsatellite analysis detected high genetic diversity (based on the number of alleles, allelic richness, and average observed heterozygosity) in the “Tōshoku” and “Shinjiko” morphotypes relative to the “Shimahire” morphotype; the “Gi-tōshoku” morphotype had an intermediate level of variation. Almost all pairwise F _ST values were significantly different from zero (P < 0.001), except between two populations of “Tōshoku.” Clear genetic independence was observed between the “Shinjiko” and “Shimahire” morphotypes in the Maruyama River. A principle component analysis based on microsatellite data indicated that the “Tōshoku,” “Shinjiko,” and “Gi-tōshoku” morphotypes were genetically similar. Furthermore, the three populations of “Tōshoku” were closely related each other, and two of those collected from the Lake Biwa system were a single population. There was, however, a high degree of genetic differentiation between “Shimahire” and the other morphotypes; moreover, there was high genetic divergence among four populations of the “Shimahire” morphotype. Amphidromous migratory histories were indicated by Sr:Ca ratios in two of three populations of the “Shinjiko” morphotype and in one of two “Gi-tōshoku” morphotypes, whereas all populations of the “Shimahire” morphotype were freshwater residents. The large genetic divergence and low genetic diversity in “Shimahire” are likely related to migration history.     

β-Adrenoceptor antagonists reinitiate meiotic maturation in Clarias batrachus oocytes

Effects of β-adrenoceptor antagonists propranolol and alprenolol in the oocyte maturation of the catfish (Clarias batrachus) were investigated under in vitro. Cyclic AMP (cAMP) levels were also measured in the control, propranolol and phosphodiesterase (PDE) inhibitor treated oocytes. When full-grown folliculated oocytes were cultured in vitro in the presence of different concentrations of propranolol or alprenolol, both the substances induced germinal vesicle breakdown (GVBD) in a dose-dependent manner. The maturational effect of alprenolol at the concentration of 1.0 mM was similar to that of the 1.5 mM dose of propranolol inducing more than 88% GVBD. In the time course study, when the oocytes were treated with 1.5 mM propranolol or with 1.0 mM alprenolol for various times, both the antagonists induced more than 80% GVBD after 4 h of incubations and this induction gradually increased with the increased duration of treatments. On the other hand, 1.5 mM propranolol treatment caused a significant decrease in oocyte cAMP which was maintained upto the duration of the study (36 h). When the oocytes were first stimulated by 1.5 mM propranolol or 1.0 mM alprenolol for 4 h and then treated with various doses of cAMP or PDE inhibitors (IBMX and theophylline), all these substances effectively blocked β-adrenoceptor antagonist-induced GVBD. Both these PDE inhibitors promoted the accumulation of cAMP in the oocytes. These results provide the first example of an existence of a cAMP-mediated mechanism of action of β-adrenoceptor antagonists in the induction of oocyte maturation in fish.     

Effects of cyclic adenosine 3′:5′-monophosphate-elevating agents and retinoic acid on differentiation in retinoid-deficient tracheal cultures

Summary The ability of cyclic AMP-elevating agents to induce normal differentiation has been investigated in retinoid-deficient hamster tracheal epithelium in organ culture. Dibutyryl cAMP (dbcAMP) and other cAMP-regulating agents alone caused disappearance of keratin and regeneration of normal mucociliary epithelium in retinoid-deficient cultures. Incubation of retinoid-deficient cultures with dbcAMP, isoproterenol, and cholera toxin (CT) (without addition of exogenous retinoid) reversed keratinization in a dose-dependent manner. The ED₅₀ of cultures treated with dbcAMP was 4×10⁻⁶ M; ED₅₀ of isoproterenol was 7×10⁻⁵ M; and CT, 0.6 μg/ml. Phosphodiesterase inhibitors and other cAMP analogs were inactive. Dibutyryl cAMP in combination with theophylline enhanced normal differentiation. Retinoid-deficient tracheas pretreated for 20 h with 10⁻⁹ M all-trans-retinoic acid (RA) responded to 10⁻⁶ M dbcAMP by potentiating normal differentiation; this concentration of dbcAMP alone was inactive. Isoproterenol showed a similar response but to a lesser degree. These cAMP-elevating agents applied in combination with theophylline did not increase activity. This investigation was supported by National Cancer Institute Contract NO1-CP-31012.     

Effects of dibutyryl adenosine 3':5'-cyclic monophosphate and other agents on induction of alkaline phosphatase activity in monkey kidney cells

The induction of alkaline phosphatase (ALP) by dibutyryl adenosine 3':5'-cyclic monophosphate (Bt2cAMP) was investigated in strain JTC-12 . P3 cells derived from monkey (Maccaca irus) kidney cortex. ALP activity was increased by Bt2cAMP in a dose-dependent manner, reaching a plateau at concentrations higher than 5 mM with the activity being about 4 times that of the controls. The concentration of Bt2cAMP required for half-maximal induction of ALP activity was about 0.8 mM. ALP activity was increased rapidly by Bt2cAMP for the first 5 days and then continued to increase gradually towards a plateau level. Removal of Bt2cAMP from the medium caused a rapid decrease in the activity, suggesting that the induction of ALP activity by Bt2cAMP is reversible. ALP activity was induced synergistically in the presence of 1 mM sodium butyrate together with Bt2cAMP at concentrations from 0.01 to 1 mM. It was also found that in the presence of 1 mM Bt2cAMP, sodium butyrate increased ALP activity in the same manner as Bt2cAMP did in the presence of 1 mM sodium butyrate. Although dexamethasone, a potent glucocorticoid, had no effect on ALP activity in control cells, the hormone suppressed the ALP activity induced by Bt2cAMP in a dose-dependent manner. At concentrations above 0.2 mM, two xanthine derivatives, theophylline and 3-isobutyl-1-methyl-xanthine (IBMX), also inhibited the induction of ALP activity by 1 mM Bt2cAMP. Inhibitors of protein synthesis, cycloheximide (1.5 micrograms/ml) and pactamycin (10 micrograms/ml), as well as inhibitors of RNA synthesis, actinomycin D (2 micrograms/ml) and alpha-amanitin (50 micrograms/ml), suppressed the induction of ALP activity.     

Effects of Nickel on Human and Fish Red Blood Cells

The objective of this study was to assess the effects of nickel chloride on human and rainbow trout erythrocytes in vitro. The cells were incubated with 0, 0.5 and 1 mM nickel chloride for 1 h at pH 7.40 and 25°C, then K⁺ efflux, SO₄²⁻ uptake and GSH and GSSG concentrations were measured. In both kind of cells, “high concentration” nickel treatment increased KCl efflux with respect to the control. The SO₄²⁻ uptake was not significantly different at “low nickel concentration” but was lower in erythrocytes treated with 1 mM nickel chloride; the rate constant of SO₄²⁻ uptake decreased by 35% in human erythrocytes and by 44% in fish erythrocytes. Nickel chloride also acts on cellular metabolism and in particular on erythrocyte glutathione peroxidase with consequent increase in oxidative stress; the data show a significant decrease in intracellular GSH in both human (25%) and fish erythrocytes (18%) after treatment with nickel chloride, with concomitantly high GSSG concentrations and lower GSH/GSSG ratios.     

Adult-type myogenesis of the frog <Emphasis Type="Italic">Xenopus laevis</Emphasis> specifically suppressed by notochord cells but promoted by spinal cord cells in vitro

Larval-to-adult myogenic conversion occurs in the dorsal muscle but not in the tail muscle during Xenopus laevis metamorphosis. To know the mechanism for tail-specific suppression of adult myogenesis, response character was compared between adult myogenic cells (Ad-cells) and larval tail myogenic cells (La-cells) to a Sonic hedgehog (Shh) inhibitor, notochord (Nc) cells, and spinal cord (SC) cells in vitro. Cyclopamine, an Shh inhibitor, suppressed the differentiation of cultured Ad (but not La) cells, suggesting the significance of Shh signaling in promoting adult myogenesis. To test the possibility that Shh-producing axial elements (notochord and spinal cord) regulate adult myogenesis, Ad-cells or La-cells were co-cultured with Nc or SC cells. The results showed that differentiation of Ad-cells were strongly inhibited by Nc cells but promoted by SC cells. If Ad-cells were “separately” co-cultured with Nc cells without direct cell–cell interactions, adult differentiation was not inhibited but rather promoted, suggesting that Nc cells have two roles, one is a short-range suppression and another is a long-range promotion for adult myogenesis. Immunohistochemical analysis showed both notochord and spinal cord express the N-terminal Shh fragment throughout metamorphosis. The “spinal cord-promotion” and long-range effect by Nc cells on adult myogenesis is thus involved in Shh signaling, while the signaling concerning the short-range “Nc suppression” will be determined by future studies. Interestingly, these effects, “Nc suppression” and “SC promotion” were not observed for La-cells. Situation where the spinal cord/notochord cross-sectional ratio is quite larger in tadpole trunk than in the tail seems to contribute to trunk-specific promotion and tail-specific suppression of adult myogenesis during Xenopus metamorphosis.     

HMG-CoA reductase and PPARα are involved in clofibrate-induced apoptosis in human keratinocytes

Contrasting data have been reported on the effects of clofibrate, a PPARα agonist and hypolipidemic drug. The carcinogenic and anti-apoptotic effects have been demonstrated especially in rodents in both “in vivo” and “in vitro” experiments. In contrast, in rat and human hepatoma cell lines, several reports have shown its concentration-dependent pro-apoptotic effect. No epidemiological data exist about its carcinogenetic effect in man. This study shows that clofibrate also induced apoptosis in a human non-tumour cell line, NCTC 2544, which shares the characteristic of proliferation with tumour cells. Both HMG-CoA reductase and PPARα were found to be involved in the signal transduction pathway inducing apoptosis, the former being the principal target: HMG-CoA reductase decreased and PPARα increased. Changes in HMG-CoA reductase expression caused activation of parameters leading to apoptosis via the mitochondria pathway. Clofibrate must be considered a pro-apoptotic molecule at concentrations of 0.25 mM and above: the effect is exercised not only on tumour cells but also on normal human proliferating cells. Clofibrate should thus be regarded as a potential drug to reduce the number of proliferating cells in pathological conditions.     

Laminin provides a better substrate than fibronectin for attachment,growth, and differentiation of 1003 embryonal carcinoma cells

Summary Culture of cells in hormonally defined media has allowed (a) the demonstration of physiological responses from cells usually unable to express them in vitro and (b) the study of the effects on growth and differentiation of diffusible factors and attachment factors. The embryonal carcinoma line 1003 forms multidifferentiated tumors in vivo but is unable to differentiate in vitro when grown in serum-containing medium. In a defined medium containing insulin, transferrin, selenium, and fibronectin as attachment factors, 1003 cells grow for several generations and differentiate into neurons and embryonic mesenchyme (Darmon et al., 1981, Dev. Biol. 85: 463–473). In the present work the effects of fibronectin and laminin were compared. In the presence of laminin the cells attached and spread better, grew faster, and could be plated at lower densities. Neurite extension was also better under these conditions and most importantly, it was found that laminin induced an important formation of muscular tissue when the cells had been seeded at low densities. Multinucleated myotubes could be stained with antibodies directed against embryonic muscular myosin. Coating the dishes with polylysine or adding FGF or serum-spreading factor to the medium allowed growth of low-density cultures with fibronectin instead of laminin but muscular differentiation was not detected under these conditions. Addition of fibronectin to laminin-containing medium did not inhibit muscular differentiation. Presented in the symposium on Plant and Animal Physiology in Vitro at the 33rd Annual Meeting of the Tissue Culture Association, San Diego, California, June 6–10, 1982. This research was supported in part by grants from the “Centre National de la Recherche Scientifique” (LA 269), the “Délégation Générale à la Recherche Scientifique et Technique,” the Fondation pour la Recherche Médicale Fran?aise,” the “Institut National de la Santé et de la Recherche Medicale,” the “Ligue Nationale Fran?aise centre le Cancer,” and the “Fondation André Meyer.” This symposium was supported in part by the following organizations: Bellco Glass, Inc., California Branch of the Tissue Culture Association, Collaborative Research, Hana Media, Hybridtech, K C Biological, Inc., and Millipore Corporation.     

Regulatory considerations when developing biological products

The Center for Biologics Evaluation and Research, whose regulatory authority includes monoclonal antibodies, cytokines, vaccines, toxins and somatic cellular therapies, communicates to sponsors issues for consideration in the development of biological products through the publication of “Points to Consider” and “Guideline” documents. This paper summarizes the available “Points to Consider” and “Guideline” documents and outlines recommendations from these documents for characterizing the cells used to produce biological products.     

Preload-induced changes in isometric tension and [Ca<Superscript>2+</Superscript>]<Subscript>i</Subscript> in rat myocardium

Preload-induced changes of active tension and [Ca²⁺]i are “dissociated” in mammalian myocardium. This study aimed to describe the distinct effects of preload at low and physiological [Ca²⁺]_o. Rat RV papillary muscles were studied in isometric conditions at 25‡C and 0.33 Hz at 1 mM (hypo-Ca group) and 2.5 mM [Ca²⁺]_o (normal-Ca group). [Ca²⁺]_i was monitored with fura-2/AM. Increase of preload caused a rise of active tension in hypo-Ca and normal-Ca groups whereas peak fluorescence rose significantly only at low [Ca²⁺]_o. End-diastolic tension, end-diastolic level of fluorescence, time-to-peak tension, but not time-to-peak of Ca²⁺ transient, progressively increased with preload. Mechanical relaxation decelerated with preload while Ca²⁺ transient decay time decreased in the initial phase and increased in the late phase, resulting in a prominent “bump” configuration. The “bump” was assessed as a ratio of its area to the fluorescence trace area. It was a new finding that the preload-induced rise of this ratio was twice as large in hypo-Ca. Our results indicate that preload-induced changes in active tension and [Ca²⁺]_i are “dissociated” in rat myocardium, with relatively higher expression at low [Ca²⁺]_o. Ca-dependence of Ca-TnC association/dissociation kinetics is thought to be a main contributor to these preload-induced effects.