Activation heat and latency relaxation in relation to calcium movement in skeletal and cardiac muscle

Measurements of activation heat, initial heat, twitch tension, and latency relaxation were made using thin-layer, vacuum-deposited thermopiles and isometric force transducers, respectively. Experiments were performed on frog skeletal muscle fiber bundles and on rabbit right ventricular papillary muscles at 0, 15, ans 21 degrees C in normal and 1.75X to 2.5X mannitol hyperosmotic bathing solutions. In skeletal muscle, activation heat, obtained by stretching to zero overlap, was only slightly affected by 1.75X hyperosmotic solution and consisted of a fast and a slow component. Both components have a refractory period and a relatively refractory period which can be demonstrated by double pulse stimulation. The twitch potentiators Zn2+ and caffeine increase the total activation heat and the magnitude and rate of the fast component. The temporal relation between the latency relaxation and activation heat is demonstrated. The latency relaxation is independent of the number of sarcomeres in series in a muscle. Activation heat and latency relaxation records from heart muscle are obtained in 2.5X hyperosmotic bathing solution. A model of excitation--contraction coupling is presented which indicates that (1) the downstroke of the latency relaxation monitors the functioning of the Ca2+-permeability or debinding mechanism in the terminal cisternae, (2) the fast component of activation heat monitors the amount of Ca2+ bound to troponin C, and (3) the total amplitude of activation heat is a measure of the total quantity of Ca2+ cycled in a twitch.     

Regional differences in the force-frequency relation of human left ventricular myocardium in mitral regurgitation: implications for ventricular shape

Sphericalization of the left ventricular (LV) chamber shape in patients with mitral regurgitation (MR) contributes to increased LV wall stress and energy consumption. On the basis of previous observations, we hypothesized the existence of regional differences in the force-frequency relation (FFR) within the LV that may contribute to its shape. Accordingly, in the present study, we assessed regional variation in the FFR in patients undergoing surgery for chronic, nonischemic MR with class II-III heart failure symptoms and related our findings to the in vivo LV shape. FFRs (steady-state isometric twitches, 0.2-3.4 Hz, 37 degrees C) were evaluated in MR myocardium from the LV subepicardial free wall (MR-FW) and papillary muscle (MR-PM) and from the subepicardial free wall in coronary artery bypass graft patients with normal LV contraction patterns [nonfailing (NF)]. Ascending slope, optimal stimulation frequency, and maximal twitch tension of the FFR were depressed in MR-FW and MR-PM compared with NF (P < 0.05). FFR depression was greater in MR-PM than in MR-FW. Between 107 and 134 beats/min, twitch tension became weaker in MR-PM, whereas it increased in MR-FW. Elevation of intracellular cAMP with forskolin eliminated FFR depression in MR-FW but not in MR-PM. MR-PM also had a 35% lower myosin heavy chain content and slowed twitch kinetics. In MR patients, the echocardiographic end-diastolic LV shape (end-diastolic eccentricity index = long axis/short axis) correlated with the ratio of ascending FFR slopes such that the end-diastolic eccentricity index increased 10% per 15% increase in slope ratio (r = 0.88, P = 0.01). These regional differences in the frequency dependence of contractility between the free wall and papillary myocardium may contribute to changes in LV shape in MR as well as during exercise.     

Determinants of energy utilization in the activated myocardium

This is a review of work dealing with the effect of pressure overload and thryotoxic hypertrophy of rabbit hearts on the production of total activity related (TA) and initial (I) heats during isometric contraction. Pressure overload hypertrophy is produced by constricting the pulmonary artery with a spiral monel metal clip. Thyrotoxic hypertrophy is produced by 14 daily i.m. injections of 0.2 mg L-thyroxine per kilogram. Heat output is measured with Hill-type planar vacuum deposited bismuth and antimony thermopiles, and force is measured with a capacitance strain gauge. The pressure overload results in a depressed velocity of unloaded shortening, a depressed rate of isometric force development, and an increased time-to-peak tension. These changes are associated with a decreased myosin ATPase, a heart with no V1 myosin isoenzyme, and an increase in the economy of isometric force development (integral of Pdt/TA, integral of Pdt/I). The thyrotoxic hearts exhibit an increased velocity of shortening and rate of force development, and a decrease in time-to-peak tension. These changes are associated with an increase in myosin ATPase activity, a heart with increase in the V1 isoenzyme composition (88% V1), and a decrease in the economy of isometric force development (integral of Pdt/TA, integral of Pdt/I). The changes in the two types of hypertrophied hearts are interpreted in terms of altered cross-bridge cycling rates and changes in cross-bridge tension time integral as well as excitation contraction coupling phenomena. In the thyrotoxic hearts there is an increase in the economy of the recovery processes. Both types of hypertrophy are considered to be adaptive and involve the coordinated restructuring of the excitation-contraction, contractile, and recovery systems.     

Host plant selection of two Mansonia blanchard species (Diptera: Culicidae) in a heterogeneous habitat of Buenos Aires City, Argentina.

Larvae and pupae of the genus Mansonia Blanchard attach to the roots of aquatic plants by means of modified structures to obtain oxygen. A study of the association of larval Ma. indubitans and Ma. titillans with floating macrophytes was conducted at Macies Pond, Argentina. Fifty-four sampling units were taken from January to May 2003. Three genera of host plants were considered: Pistia, Limnobium, and Salvinia. A total of 402 immatures of Ma. indubitans and 217 of Ma. titillans were captured and associations between Mansonia immatures and roots of each genera were assesed. Significant association was noted between Ma. indubitans and certain host plant species (K-W H=42.74, df=2, p<0.001). The same result was observed for Ma. titillans (K-W H=23.42, df=2, p<0.001). Both Mansonia species utilized roots of P. stratiotes in significantly higher proportions than expected by random selection. Both species showed significant negative association with Salvinia spp., while no clear relationships were detected with L. laevigatum.     

Heat, mechanics, and myosin ATPase in normal and hypertrophied heart muscle

In this paper we review our previous work on the myothermic economy of isometric force production in compensated cardiac hypertrophy secondary to pulmonary artery constriction (pressure overload) and/or thyrotoxicosis (volume overload). Hypertrophy-induced changes in isotonic and isometric twitch mechanics are correlated with accompanying changes in actin-activated myosin ATPase and heat liberation. Heat measurements were made with rapid, high-sensitivity thermopiles on right ventricular papillary muscles from normal and hypertrophied rabbit hearts. Total activity-related heat was separated into initial and recovery heat. Initial heat was separated into a tension-dependent component (TDH) relating to cross-bridge activity, and a tension-independent component (TIH) relating to excitation-contraction coupling. There were oppositely directed changes in most parameters studied in pressure overload hypertrophy (P) as compared with thyrotoxic hypertrophy (T). Thus, in P there was depression (30-50% in the rate of isometric force production, mechanical Vmax, TDH and TDH rate, myosin ATPase, TIH, and prolongation in time-to-peak twitch tension, whereas in T all parameters were oppositely changed except for no change in TIH. Thyrotoxicosis following pressure overload reversed the P-induced changes in all parameters. There was a direct, linear relation between in vitro actin-activated myosin ATPase and in vivo TDH. However, TDH per unit twitch tension or tension-time integral varied inversely with ATPase, making force production more economical than normal in P muscles and less economical than normal in T muscles. These cellular changes beneficially equip P hearts for slow, high-pressure, economical pumping the T hearts for fast, high-volume, uneconomical pumping. The differences are similar to those between slow and fast skeletal muscle and between neonatal and adult skeletal muscle. The mechanism of these changes is discussed in terms of an enzyme kinetic scheme of chemomechanical coupling in actomyosin interaction.     

Winter survival of immature instars of Mansonia indubitans Dyar & Shannon and Mansonia titillans Walker (Diptera: Culicidae), in Buenos Aires, Argentina

We conducted a whole year research on the ecology of Mansonia indubitans and Ma. titillans in Macáes Pond, Costanera Sur Reserve, Buenos Aires, Argentina. The usage of different floating plants by immature instars and their overwintering was analyzed. The percentage of usage of the available floating macrophytes (Pistia, Limnobium, and Salvinia) by the larvae and pupae was studied. Also, we defined positivity (P+) as the percentage of plants with immature instars for each plant genus on a monthly basis. Ma. immature instars were captured throughout the year and Pistia was the resource most commonly exploited by the mosquitoes. The percentage of fourth-instar larvae and pupae on Pistia roots with respect to total immature instars captured was assessed on a monthly and seasonal basis. The proportion of fourth-instar larvae and pupae from both species of Mansonia on water lettuce roots, showed significant differences between months and seasons. Our results suggest that the populations of Ma. indubitans and Ma. titillans in Macáes Pond, survive during winter mainly as fourth-instar larvae.