Lars Cleemann - US grants

The aim of the proposed research is to measure extracellular Ca2+ depletion in ventricular heart muscle from frog and cat and relate it to the force development and the membrane current. Such measurements will give information about the different components of sarcolemmal Ca2+ transport and the relative importance of the different sources of activator Ca2+. In this project Ca2+ sensitive dyes are used to measure changes in the extracellular Ca2+ content during single action potentials and voltage clamp depolarizations. The optical measurements of extracellular dye absorption are performed in a single sucrose gap chamber equipped also with an isometric force transducer and a voltage clamp system: 1) The effects of drugs and ionic interventions (epinephrine, Ca2+ channel blockers, digitalis, low Ko, low Nao, low Clo) on the Ca2+ signal in frog ventricular muscle will be related to simultaneous changes in the twitch force and the action potential. 2) Voltage clamp experiments will be used in an attempt to quantify the properties of sarcolemmal Ca2+ transport mechanisms. Specific drugs will be used to suppress or enhance either the slow inward Ca current or coupled transport mechanisms (Na/Ca exchange). 3) The involvement of internal Ca stores in excitation-contraction coupling will be examined by measuring the extracellular Ca depletion which persists between beats. In frog and cat it will be examined if Ca is released from such stores during subsequent beats or voltage clamp depolarizations. This project may contribute to the basic understanding of excitation-contraction coupling in heart muscle and elucidate Ca2+ transport mechanisms mediating the actions of cardiac drugs.

DESCRIPTION (provided by applicant): We propose to develop total internal reflectance fluorescence (TIRF) microscopy in an imaging modality with a sharper focal plane (approximately 60 nm) and sub-millisecond time resolution (2000 frames/sec) for the study of rapid signaling near the cell membrane. To achieve this we shall use a novel optical design and operate in a mode where the response time of the fluorescent probe (e.g. Fluo- 3) is governed primarily by its diffusion out of the evanescent field (<<1 ms), not by its association time (k/off/-1 approximately- 10 ms). Preliminary experiments performed with a prototype system (approximately 100 nm focal plane, 734 frames/sec) have demonstrated proof-of-principle and resolved subsarcolammel Ca 2v signals in voltage-clamped rat atrial cardiomyocytes in terms of apparent single-channel Ca 2+ fluxes and intermittently active Ca 2+ release-components of Ca 2+ sparks. The specific aims are: -1-To sharpen the focal plane (100->60 nm) by using denser optical materials (n: 151->1.77) and purchase and use a faster, more sensitive CCD camera (734-> 2000 frames/sec). -2- To evaluate the performance of this system with respect to speed, sensitivity and noise while varying the penetration (200-60 nm) and using diffusible and non-diffusible fluorescent probes. -3-To ascertain if the subsarcolemmal Ca2+-signals in atrial cardiomyocytes can be resolved in terms of single channel Ca 2+ fluxes produced by DHP- and/or ryanodine-receptors. -4-To evaluate, in a model system of cultured adrenal chromaffin cells, if protons are co-released in sufficient numbers to modulate synaptic transmission by transient acidification of synaptic clefts. While the two listed applications are relevant to our ongoing research, we believe that implementation of the TIRF technique with sufficient sensitivity and speed to match electrical recordings of single-channel currents will be of considerable general usefulness in the study of the structures and regulatory processes that are associated with cell membranes and their immediate vicinity.