Identification and quantification in single muscle fibers of four isoforms of parvalbumin in the iliofibularis muscle of Xenopus laevis

Is high concentration of parvalbumin a requirement for superfast relaxation?

It is generally thought that the rapid relaxation of fast muscles is facilitated by the Ca(2+) binding protein parvalbumin (Parv). Indeed superfast swimbladder (SWB) muscle of toadfish contains the largest concentration of this protein ever observed (up to 1.5 mM). At 15 degrees C toadfish perform a 100 Hz call, 400 ms in duration, followed by a long (5-15 s) intercall interval. It has been proposed that Parv helps sequester the Ca(2+) during the call, and then Ca(2+) unbinds and is pumped back into the sarcoplasmic reticulum during the long intercall interval. Midshipman (Porichthys notatus) is another fish which calls at a high frequency; 80-100 Hz at a temperature of 12-15 degrees C. However, unlike toadfish, midshipman call with a 100% duty cycle. Without an intercall interval, Parv would seem of little use as it would become saturated early in calling. Here we show that the midshipman SWB has only about 1/8th of the Parv in toadfish. Moreover, total Parv content in calling male midshipman SWB was not different from that in the non-calling female and the much slower locomotory muscles. These data suggest that Parv does not play a large role in the calling of midshipman, which is accomplished without a high concentration of this protein. Native gel-electrophoresis also revealed presence of three major (PA-I, PA-II and PA-III) and two minor (PA-Ia and PA-IIIa, <5% of total content) Parv isoforms in adult toadfish SWB. Midshipman SWB contained about equal amounts of PA-I and PA-II and also a small (approximately 10%) amount of PA-III. By amino acid composition, toadfish PA-Ia and PA-I isoforms were different from PA-II and PA-III isoforms (by 24 and 14 residues, respectively).

Parvalbumin content in striated muscles of the common shrew (Sorex araneus)

The parvalbumin content of mammalian muscles correlates positively with isometric relaxation rate and fiber type IIB frequency of the muscles but negatively with animal size. Since shrews are small-bodied animals with a relatively low number of type IIB fibers, it is of some interest to know how the parvalbumin content of shrew muscle correlates with the above factors. Parvalbumin content in heart, diaphragm, and gastrocnemius muscle of the common shrew, mouse, and rat was determined electrophoretically. Parvalbumin was not found in heart muscle of any species. Shrew diaphragm (0.29 ± 0.04 g/kg) had significantly less parvalbumin than mouse (0.63 ± 0.11 g/kg) or rat (0.54 ± 0.09 g/kg) diaphragm. Similarly, the parvalbumin content of shrew gastrocnemius muscle (0.28 ± 0.04 g/kg) was significantly lower than in that of mouse (2.88 ± 0.38 g/kg) or rat (0.96 ± 0.25 g/kg) gastrocnemius muscle. The isometric twitch of the gastrocnemius muscle was somewhat faster than the twitch of the diaphragm in all three species. The isometric contractions of shrew and mouse skeletal muscles were generally very similar in duration, with the exception of the relaxation time of the gastrocnemius muscle, which was shorter in the mouse. Diaphragm and gastrocnemius muscle of the rat were clearly slower than the respective muscles in the mouse or shrew with regard to both the contraction and relaxation phases. The half-relaxation time of isometric contractions correlated relatively weakly with parvalbumin content of the muscles (r = 0.40) but more strongly with their fiber IIB content (r = 0.81). The unexpectedly low parvalbumin content and relatively slow rate of contraction in shrew skeletal muscles are attributed to the exceptional fiber type composition, i.e., a high proportion of type IID fibers.

The isolation of parvalbumin isoforms from the tail muscle of the American alligator (Alligator mississipiensis)

Multiple parvalbumin isoforms have been detected in the tail (skeletal) muscle of the American alligator (Alligator mississipiensis). One of these isoforms (APV-1) has been highly purified and partially characterized. Protein purification involved mainly gel filtration and anion exchange chromatography, and characterization included gel electrophoresis, amino acid composition analysis, metal ion analysis, MALDI-TOF and ESI mass spectrometry, ultraviolet and fluorescence spectroscopy, and one- and two-dimensional 500 MHz proton NMR spectroscopy. The alligator isoforms are rich in phenylalanine and deficient in the other aromatic residues as is typical for parvalbumins. In fact, the one highly purified isoform that forms the basis of this study has only phenyl-alanine as an aromatic residue. Ion exchange chromatography further indicates that this isoform has a relatively high isoelectric point (pl approximately 5.0), indicating that it is an alpha-lineage parvalbumin. This alligator parvalbumin isoform is unusual in that it has an atypically high Ca2+ content (almost 3.0 mole of Ca2+ per mole of protein) following purification, a fact supported by terbium fluorescence titration experiments. Preliminary comparative analysis of the highly purified alligator parvalbumin isoform (in the Ca2-loaded state) by two-dimensional 1H-NMR (2D 1H TOCSY and 2D 1H NOESY) indicates that there is considerable similarity in structure between the alligator protein and a homologous protein obtained from the silver hake (a saltwater fish species).

Oncomodulin is abundant in the organ of Corti

A small, acidic Ca(2+)-binding protein (CBP-15) was recently detected in extracts of the mammalian auditory receptor organ, the organ of Corti [Senarita et al. (1995) Hear. Res. 90, 169-175]. N-terminal sequence data for CBP-15 [Thalmann et al. (1995) Biochem. Biophys. Res. Commun. 215, 142-147] implied membership in the parvalbumin family and possible identity with the mammalian beta-parvalbumin oncomodulin. As shown herein, the latter conclusion is supported by strong cross-reactivity between CBP-15 and isoform-specific antibodies to oncomodulin. Moreover, we have succeeded in amplifying the guinea pig CBP-15 coding sequence from organ of Corti cDNA using degenerate oligonucleotide primers based on the rat oncomodulin sequence. The deduced amino acid sequence of guinea pig CBP-15 displays 90%, 92%, and 98% identity with mouse, rat, and human oncomodulin isoforms. Demonstration of the presence of oncomodulin in the organ of Corti is the first documentation of this substance in a postnatal mammalian tissue.

Taste buds develop autonomously from endoderm without induction by cephalic neural crest or paraxial mesoderm

Although it had long been believed that embryonic taste buds in vertebrates were induced to differentiate by ingrowing nerve fibers, we and others have recently shown that embryonic taste buds can develop normally in the complete absence of innervation. This leads to the question of which tissues, if any, induce the formation of taste buds in oropharyngeal endoderm. We proposed that taste buds, like many specialized epithelial cells, might arise via an inductive interaction between the endodermal epithelial cells that line the oropharynx and the adjacent mesenchyme that is derived from both cephalic neural crest and paraxial mesoderm. Using complementary grafting and explant culture techniques, however, we have now found that well-differentiated taste buds will develop in tissue completely devoid of neural crest and paraxial mesoderm derivatives. When the presumptive oropharyngeal region was removed from salamander embryos prior to the onset of cephalic neural crest migration, taste buds developed in grafts and explants coincident with their appearance in intact control embryos. Similarly, explants from neurulae in which movement of paraxial mesoderm had not yet begun also developed taste buds after 9–12 days in vitro. We conclude that neither cranial neural crest nor paraxial mesoderm is responsible for the induction of embryonic taste buds. Surprisingly, the ability to develop taste buds late in embryonic development seems to be an intrinsic feature of the oropharyngeal endoderm that is determined by the completion of gastrulation.

Force-dependent and force-independent heat production in single slow- and fast-twitch muscle fibres from Xenopus laevis

1. The origin of labile heat production, i.e. a heat component which rapidly decays after the onset of stimulation, and of stable (maintenance) heat production was investigated in intact single fast-twitch (type 1) and slow-twitch (type 3) iliofibularis muscle fibres from Xenopus laevis, at 20 degrees C, by varying stimulation frequency and by varying sarcomere length and the concentration of 2,3-butanedione 2-monoxime (BDM) added. 2. The labile heat produced consisted of a force-independent and a force-dependent part. The average parvalbumin (PA) content found in type 1 fibre bundles (0.84 +/- 0.08 mM; mean +/- S.E.M.; n = 5) and in type 3 fibre bundles (0.12 +/- 0.02 mM; n = 5) indicates that the force-independent labile heat is explained by Ca(2+)-Mg2+ exchange on PA, and amounts to a molar enthalpy change of -78 kJ (molPA)-1. 3. Force-dependent labile heat during fused contractions was similar to the calculated heat production resulting from the formation of force-generating cross-bridges, assuming an enthalpy change associated with cross-bridge formation of -30 kJ mol-1. 4. Activation heat, i.e. the part of the total stable heat that is not related to the contractile apparatus, and of which the calcium sequestration by the sarcoplasmic reticulum is the most important contributor, determined by varying sarcomere length or BDM concentration, was identical. For fused contractions the fraction activation heat of the stable maintenance rate of heat production was 34 +/- 4% (mean +/- S.E.M.; n = 13) in type 1 fibres, and 52 +/- 4% (n = 15) in type 3 fibres. In unfused contractions this was 48 +/- 5% (n = 13) in type 1 fibres, and 35 +/- 2% (n = 11) in type 3 fibres. 5. From the force-dependent stable rate of heat production the economy of cross-bridge cycling, expressed as the force-time integral for a single myosin head per ATP molecule hydrolysed, was calculated. It followed that cross-bridge interaction in type 3 fibres is more economical than in type 1 fibres, and that fused contractions are more economical than unfused contractions.

Slowing of relaxation and [Ca2+]i during prolonged tetanic stimulation of single fibres from Xenopus skeletal muscle

1. Parvalbumin (PA) has been proposed to take up Ca2+ and enhance skeletal muscle relaxation in brief contractions; as the duration of the contraction is increased, PA will become saturated with Ca2+ and no longer contribute to relaxation which therefore will be slowed. The rate of Ca2+ loading of PA is determined by the Mg2+ off rate (about 4 s-1 at 22 degrees C). In the present study we produced prolonged tetani in intact, single fibres of Xenopus frogs while measuring force and the free myoplasmic [Ca2+] ([Ca2+]i) with indo-1. 2. Mean rate constants of slowing of force relaxation with increasing tetanus duration ranged between 3.2 and 4.8 s-1, thus, similar to the Mg2+ off rate of PA. 3. The amplitude of the tail of [Ca2+]i after tetani increased with tetanus duration. This increase developed with a rate constant similar to the Mg2+ off rate of PA 4. Steady-state force-[Ca2+]i curves were produced from tetani of various frequencies and tetani produced when force was depressed after fatiguing stimulation. These curves were used to convert [Ca2+]i records into Ca(2+)-derived force. Relaxation of Ca(2+)-derived force was slowed following a time course similar to that of real force. The lag between Ca(2+)-derived and real force during relaxation was not affected by tetanus duration. 5. Tails of elevated [Ca2+]i after tetani were used to analyse the function of the SR Ca2+ pumps. This analysis showed a marked decline in the rate of Ca2+ uptake with prolonged tetani. 6. In conclusion, in Xenopus fibres the slowing of relaxation with increasing tetanus duration can be explained by altered Ca2+ handling due to PA Ca2+ loading and impaired SR Ca2+ uptake. This contrasts to our previous results in mouse fibres and the difference can be explained by a markedly lower rate of SR Ca2+ uptake resulting in higher tetanic [Ca2+]i in Xenopus fibres.

Force relaxation, labile heat and parvalbumin content of skeletal muscle fibres of Xenopus laevis

1. Measurements were made of stable (hb) and labile (ha) maintenance heat rate, slowing of relaxation as a function of tetanus duration, and parvalbumin (PA) content in intact single muscle fibres of types 1 and 2 from Xenopus laevis. The majority of experiments were performed at 20 degrees C. In addition, total and myofibrillar ATPase activity was measured in skinned Xenopus fibres, also of types 1 and 2; these studies were performed at 4 degrees C. 2. In agreement with a previous study hb was significantly higher in type 1 (175 +/- 13 mW (g wet wt)-1; n = 8) than in type 2 fibres (88 +/- 9 mW (g wet wt)-1; n = 7). The value of ha was 236 +/- 22 and 117 +/- 16 mW (g wet wt)-1, respectively (mean +/- S.E.M.). ha decayed with a time constant of 0.27 +/- 0.02 (n = 8) and 0.33 +/- 0.02 s (n = 7). 3. The early relaxation rate of tetanic force, extrapolated to the onset of stimulation (yo + yb; where yo is 'extra' rate of relaxation and yb steady rate) was 85.6 +/- 4.2 s-1 for type 1 fibres (n = 8) and 62.7 +/- 7.3 s-1 for type 2 fibres (n = 7). Relaxation rate at the end of a 1.8 s tetanus (yb) was 29.4 +/- 1.6 and 33.3 +/- 1.5 s-1, respectively; thus, there was more slowing with tetanus duration in type 1 fibres. The time constant for slowing of relaxation with tetanus duration was similar to that for decay of ha. 4. Parvalbumin concentration, [PA], was 0.45 +/- 0.04 mM in type 1 (n = 7) and 0.22 +/- 0.04 mM (n = 7) in type 2 fibres. 5. For individual fibres positive correlations were found between the 'extra' rate of relaxation (yo), labile heat (ha) and [PA]. Significantly more labile heat was liberated than can be accounted for by the enthalpy change of Ca2+ binding to PA. 6. For five fibres (type 1) studied both at 20 and 10 degrees C, the magnitude of slowing of relaxation, expressed as yo/(yo + yb), was 0.58 +/- 0.03 at 20 degrees C and 0.65 +/- 0.03 at 10 degrees C. 7. Both slowing of relaxation and labile heat were depressed in the second of two closely spaced tetani in type 1 fibres. Repriming of both effects followed similar, biphasic time courses and required more than 10 min for completion at 20 degrees C.(ABSTRACT TRUNCATED AT 400 WORDS)

Relaxation rate of intact striated muscle fibres after flash photolysis of a caged calcium chelator (diazo-2)

Single twitch fibres from lumbrical muscles of Xenopus have been loaded with the photolysable calcium-chelator diazo-2 by incubation in Ringer solution containing the membrane permeable acetoxymethyl ester (AM) form of diazo-2. Incubation caused a progressive slowing of tetanus rise and relaxation which is ascribed to calcium-buffering by unphotolyzed diazo-2 (Kd = 2.2 microM). After incubation, exposure to a brief UV flash caused a three to four fold increase in the rate of tension fall. A flash given 16-18 ms after the last tetanic stimulus (at 22-24 degrees C) resulted in 10% increase in relaxation rate compared with the control before incubation. A much bigger effect was observed when a flash was given half-way into the slow phase, where an 1.8-1.9-fold increase in relaxation rate, above the preincubation slope, was observed. It is concluded that rapid lowering of [Ca]i, and hence more rapid removal of Ca2+ from troponin, speeds up relaxation, indicating that calcium translocation is the major determinant of the rate of tension fall during the isometric phase of relaxation.

Neural regulation of calmodulin in adult Xenopus leg muscle

Two Ca(2+)-binding proteins important in regulating muscle responses to Ca2+ flux are differentially expressed following denervation of Xenopus laevis gastrocnemius. Levels of parvalbumin (PV) RNA transcripts and proteins decrease in abundance, while calmodulin (CaM) transcript and protein levels increase. Our studies on PV kinetics in Xenopus follow a pattern observed in other species, however, our observation of a concomitant increase in CaM has not been documented in any system. Molecular analyses of the Xenopus CaM gene indicate that its structure and upstream sequences are highly conserved across several vertebrate species and implicate several transcription factors in the regulation of its expression.

Identification of a novel parvalbumin in avian thymic tissue

A novel calcium-binding protein has been isolated from chicken thymus tissue. Its molecular weight (approximately 11,500) and characteristic interactions with Tb3+ and Eu3+ identify the protein as a member of the parvalbumin family. Electrophoretically distinct from both chicken (muscle) parvalbumin and avian thymic hormone, it represents the third parvalbumin to be identified in avian tissues and the second to be identified in the avian thymus gland.

Avian thymic hormone and chicken (muscle) parvalbumin are distinct proteins: isolation of a muscle parvalbumin cDNA fragment by PCR

Access to the nucleotide sequence of parvalbumin from chicken muscle was gained via the polymerase chain reaction. In the absence of specific amino acid sequence data, the PCR primers were based on consensus data for the two parvalbumin Ca2(+)-binding sites. The 137 bp fragment obtained by amplification clearly codes for a parvalbumin, as judged by the presence of 10 invariant codons within the sequence flanked by the primers. When used to probe poly(A)+ RNA from chicken muscle, the fragment recognizes an 800 nucleotide transcript. The translated nucleotide sequence of the muscle protein is unmistakably distinct from that of the thymus-specific parvalbumin known as avian thymic hormone. Of the 31 amplified residues, the two proteins differ at 14. The presence of a distinct parvalbumin in chicken thymus is consistent with the potent effector role proposed for the protein.

Slowing of relaxation during fatigue in single mouse muscle fibres

1. In the preceding paper we analysed the force decline in fatigue of isolated mouse muscle fibres. Using the same preparation and stimulation scheme we have now examined another aspect of muscle fatigue, namely slowing of relaxation. 2. Relaxation at the end of a tetanic contraction can usually be divided into two phases, an initial nearly linear force decline, followed by an exponential phase. We have analysed the initial phase in terms of decline rate and duration. In rested fibres the decline rate after a 350 ms tetanus was not affected by a 30% reduction of tetanic tension (obtained by decreasing the stimulation frequency). 3. Relaxation became gradually slower during fatiguing stimulation with a maximum reduction at the time when tetanic tension was reduced to about 75% of the original (end of phase 2, see preceding paper). At this time the decline rate of the linear phase had fallen to 55.2 +/- 2.9% (mean +/- S.E.M., n = 25) and its duration had increased to 151.3 +/- 14.2% of the control (unfatigued 350 ms, 70 Hz tetanus). 4. Short rest periods (duration = 10s), given at various times during fatiguing stimulation, resulted in a clear, but partial, normalization of the relaxation parameters; at the time of maximum slowing the mean decline rate increased from 50.3 to 58.7% and the duration decreased from 167.9 to 144.0% of the control (n = 14). 5. The influence of intracellular acidosis on relaxation was studied by exposing rested fibres to 30% CO2 instead of the normal 5%. This resulted in a decline rate of 43.0 +/- 2.6% and a duration of 221.1 +/- 13.1% of the control (n = 7). 6. In amphibian muscle relaxation is known to become gradually slower during a single, prolonged tetanus. The existence of such an effect also in the present preparation was studied by giving 'interrupted' tetani with a total duration of about 2 s. In rested fibres the mean rate of relaxation was found to fall from 140.9 to 71.8% (n = 11) of the control (end of 350 ms stimulation) with a time constant of about 0.5 s. Thus, a marked slowing during a long tetanus occurs also in mammalian muscle. 7. A distinct slowing of relaxation during prolonged tetani was observed also in the fatigued and in the acidified state. Under these two conditions the mean rate of relaxation fell from 87.0 to 34.0% (n = 3) and from 74.0 to 23.0% (n = 5) of the control, respectively, with time constants similar to that in the rested state.(ABSTRACT TRUNCATED AT 400 WORDS)

Molecular cloning of the thymus-specific parvalbumin known as avian thymic hormone: isolation of a full length cDNA and expression of the recombinant protein in Escherichia coli

The complete coding sequence of the thymus-specific parvalbumin called avian thymic hormone (ATH) has been cloned into Escherichia coli. The translated amino acid sequence was found to be identical to the sequence of map turtle parvalbumin at 90 of 108 positions. Northern blot analysis of thymic RNA indicated a transcript length of approximately 1050 bp. However, the ATH cDNA probe failed to hybridize to poly(A)+ RNA from chicken leg muscle, a further indication that avian thymic hormone is distinct from the muscle-associated parvalbumin previously isolated from chicken. Southern analysis of chicken genomic DNA suggests the presence of a single copy of the ATH gene, and the absence of hybridization between an ATH cDNA fragment and genomic DNA from rat and rabbit is confirmatory evidence that ATH expression is restricted to avian species. One of the full length ATH cDNA clones harbored an insert that lacked all 5' noncoding sequences. This cDNA was inserted without further alteration into the prokaryotic expression vector, pKK223-3. The resulting construction, which contains eleven base pairs between the Shine-Dalgarno sequence and the initiation codon, affords reasonably high levels of expression in E. coli. In most respects, recombinant ATH mimics the tissue-derived protein, retaining a similarly high affinity for Ca2+ ion (KCa = 14 +/- 5 nM). However, in contrast to ATH isolated from chicken thymus tissue, the N-terminal alanine of recombinant ATH is unacetylated. As a result, the isoelectric point is shifted upward from 4.3 to approximately 4.8.

An assay for sarcoplasmic reticulum Ca2(+)-ATPase activity in muscle homogenates

A spectrophotometric method is described for the determination of sarcoplasmic reticulum (SR) Ca2(+)-ATPase activity (EC 3.1.6.38) in unfractionated muscle homogenates. Conditions were established that give maximal SR Ca2(+)-ATPase activity, while eliminating Ca2(+)-dependent myofibrillar ATPase activity and reducing Ca2(+)-independent or background ATPase activity. High [Ca2+] (20 mM) could be used to selectively inhibit the SR Ca2+ ATPase. Identification of the Ca2(+)-dependent ATPase activity in muscle homogenates as being SR Ca2+ ATPase was based on a comparison of several parameters using homogenate material and purified SR. The following parameters were compared and found to be the same in homogenate and SR: activation and inactivation between 0 and 20 mM Ca2+, temperature dependence, sensitivity toward Triton X-100, and the maximal level of inhibition of ATPase activity achieved by an antibody specific for SR Ca2+ ATPase. The method is illustrated with the analysis of homogenates prepared from freeze-dried muscle fibers and thin sections of muscles typically used in microscope analyses as well as an analysis of freshly prepared homogenates from various types of muscle, which shows a good correlation over a wide range between SR specific Ca2(+)-uptake and -ATPase activities. In addition, a simple, easily constructed cuvette is described which allows the analysis of less than 5 micrograms of tissue (wet weight) in a volume of 25 microliters.