Heat production in chemically skinned smooth muscle of guinea-pig taenia coli

Biomedical use of isothermal microcalorimeters

Isothermal microcalorimetry is becoming widely used for monitoring biological activities in vitro. Microcalorimeters are now able to measure heat production rates of less than a microwatt. As a result, metabolism and growth of relatively small numbers of cultured bacteria, protozoans, human cells and even small animals can be monitored continuously and extremely accurately at any chosen temperature. Dynamic effects on these organisms of changes in the culture environment—or of additions to it—are easily assessed over periods from hours to days. In addition microcalorimetry is a non-destructive method that does not require much sample preparation. It is also completely passive and thus allows subsequent evaluations of any kind on the undisturbed sample. In this review, we present a basic description of current microcalorimetry instruments and an overview of their use for various biomedical applications. These include detecting infections, evaluating effects of pharmaceutical or antimicrobial agents on cells, monitoring growth of cells harvested for tissue eingineering, and assessing medical and surgical device material physico-chemical stability and cellular biocompatibility.

Time-resolved measurements of phosphate release by cycling cross-bridges in portal vein smooth muscle

The rate of release of inorganic phosphate (Pi) from cycling cross-bridges in rabbit portal-anterior mesenteric vein smooth muscle was determined by following the fluorescence of the Pi-reporter, MDCC-PBP (Brune, M., J. L. Hunter, S. A. Howell, S. R. Martin, T. L. Hazlett, J. E. T. Corrie, and M. R. Webb. 1998. Biochemistry. 37:10370-10380). Cross-bridge cycling was initiated by photolytic release of ATP from caged-ATP in Triton-permeabilized smooth muscles in rigor. When the regulatory myosin light chains (MLC20) had been thiophosphorylated, the rate of Pi release was biphasic with an initial rate of 80 microM s-1 and amplitude 108 microM, decreasing to 13.7 microM s-1. These rates correspond to fast and slow turnovers of 1.8 s-1 and 0.3 s-1, assuming 84% thiophosphorylation of 52 microM myosin heads. Activation by Ca2+-dependent phosphorylation subsequent to ATP release resulted in slower Pi release, paralleling the rate of contraction that was also slower than after thiophosphorylation, and was also biphasic: 51 microM s-1 and 13.2 microM s-1. These rates suggest that the activity of myosin light chain kinase and phosphatase ("pseudo-ATPase") contributes <20% of the ATP usage during cross-bridge cycling. The extracellular "ecto-nucleotidase" activity was reduced eightfold by permeabilization, conditions in which the ecto-ADPase was 17% of the ecto-ATPase. Nevertheless, the remaining ecto-ATPase activity reduced the precision of the estimate of cross-bridge ATPase. We conclude that the transition from fast to slow ATPase rates reflects the properties and forces directly acting on cross-bridges, rather than the result of a time-dependent decrease in activation (MLC20 phosphorylation) occurring in intact smooth muscle. The mechanisms of slowing may include the effect of positive strain on cross-bridges, inhibition of the cycling rate by high affinity Mg-ADP binding, and associated state hydrolysis.

Direct microcalorimetry as a technique in cell cultures

In vitro studies of energy metabolism in isolated cells contribute to improved knowledge of human energy metabolism under normal and pathological conditions. In every cellular system energy is taken up, metabolized and finally transformed into heat, which is dissipated into the environment. Thus, energy turnover of isolated cells can be assessed by microcalorimetric determination of their heat production. Microcalorimeters of the thermopile heat conduction type facilitate direct physical determination of thermogenesis with a sensitivity of 0.2 microW; 10(4)-10(5) cells being sufficient for one measurement. Peltier elements are sandwiched between the sample and a precisely thermostated heat sink, creating a detectable voltage proportional to the heat production. For adequate interpretation of the results, simultaneous biochemical investigations of relevant metabolic pathways are required. Up to now, numerous studies with blood cells, skeletal and heart muscle cells, hepatocytes, endothelial cells, fibroblasts and adipocytes have been performed in relation to various diseases and under the influence of certain hormones and pharmacological agents.

Smooth muscle energetic in the pig coronary artery during a calcium-dependent and a calcium-independent isometric contraction

Heat production by resting smooth muscle, was measured with a heat-flux micro calorimeter on cut-open segments of pig coronary artery superfused at 30 degrees C, was 0.93+/-0.06 (n=16) mW/g wet weight. Time courses of the increases in isometric tension and heat production with respect to basal during sustained supra maximal acetylcholine stimulation were qualitatively similar: initial peak tapering down to a supra basal plateau. Mean tension-associated heat production over 1 h was 0.16 J/g. During sustained exposure to phorbol 12,13-dibutyrate, supra basal tension and--with a 5-10 min initial delay--supra basal heat progressively increased to a plateau in about 40 min. Mean tension-associated heat production over 1 h was only 0.02 J/g with normal extracellular and intracellular mobilizable Ca2+ pools, and it was further reduced to 0.01 J/g with depleted Ca2+ pools. These results show that the maintenance--if not necessarily the building up--of tension under phorbol 12,13-dibutyrate does not entail any large dissipation of energy and is not dependent on the presence of normal Ca2+ pools.

Force development with inosine triphosphate and uridine triphosphate in chemically skinned vascular smooth muscle

The contraction of vascular smooth muscle is thought to be regulated by reversible phosphorylation of the 20,000 dalton light chains of myosin, catalyzed by myosin light chain kinase that is dependent on calcium and calmodulin. With phosphorylation, there is a coincident increase in the actin-activated myosin NTPase activity, cross bridge interaction and contractile activity. However, this myosin phosphorylation mechanism may not be the sole factor controlling actin-myosin interaction in vascular smooth muscle. Other mechanisms may function in addition to this myosin-linked regulation. A calcium-insensitive regulation of contraction was observed in helical strips of chemically skinned (Triton X-100) arterial smooth muscle. Millimolar concentrations of inosine triphosphate and uridine triphosphate supported concentration dependent force development in the absence of calcium. Force development was a function of the MgNTP concentration. At high free calcium concentrations, an additional component of force was observed. ITP and UTP, in contrast to ATP, are less effective substrates for the myosin light chain kinase, and their effect on actin-myosin interaction is thus less than that of ATP. They are, however, utilized by the myosin NTPase after treatment by ATP-gamma-S. The efficacy of the substrate for the activated NTPase is greater for UTP than ITP than for ATP.

Dependence of ATP consumption on cross-bridge phosphorylation in swine carotid smooth muscle

1. Ca(2+)-dependent phosphorylation of the myosin regulatory light chain (MRLC) initiates cross-bridge cycling and contraction in smooth muscle. A four-state cross-bridge model, in which Ca(2+)-dependent phosphorylation is the only proposed regulatory mechanism, can predict the mechanical output of the swine carotid media. Our aims were to determine whether ATP consumption rates and the economy of force maintenance are regulated functions of MRLC phosphorylation as predicted by the model. 2. Steady-state force and oxygen consumption were measured in medial rings of swine carotid arteries activated with depolarizing solutions and agents capable of maintaining a wide range of steady-state myoplasmic Ca2+ and MRLC phosphorylation levels. 3. Suprabasal ATP consumption increased almost linearly with MRLC phosphorylation and exhibited a hyperbolic increase with active stress, as predicted. 4. The economy of stress maintenance fell with increases in suprabasal phosphorylation. 5. In absolute terms the energetic cost of covalent regulation by cross-bridge phosphorylation was small, although it may be a significant fraction of the ATP consumption associated with contraction.