STUDY OF BEATING FREQUENCY OF A SINGLE MYOCARDIAL-CELL .2. ULTRAVIOLET MICRO-IRRADIATION OF NUCLEUS AND OF CYTOPLASM

Single rat myocardial cells were irradiated with the UV micro-irradiation technique over a nuclear or cytoplasmic area of 5 .mu.m of diameter. The contractile response was studied immediately after the irradiation. After 103 ergs mm-2 of UV light (254 nm), 4% and 21% of the cells irradiated in the nucleus and the cytoplasm, respectively, showed a temporary increase of the beating rhythm. Moreover, cytoplasmic regions rich in mitochondria were more excitable than other cytoplasmic regions. The ultrastructure and the survival of these cells 24 h after the irradiation did not differ from the control cells. The change of the contractile response according to the localization of the irradiation indicates that the main target organelles are mitochondria; the role of the membrane is not excluded when higher doses of irradiation are considered.