Photodynamic effects of erythrosine on the smooth muscle cells of guinea‐pig taenia coli

1 Photon activation of the halogenated fluorescein derivative erythrosine caused a marked calcium‐dependent contraction of the smooth muscle cells of the guinea‐pig taenia coli superfused in vitro. Neither high intensity illumination alone (up to 5 × 104 lux) nor erythrosine alone (up to 2 × 10−4 M) altered the tone of the taenia or its ability to respond to carbachol (5 × 10−5M); photo‐irradiation of erythrosine before tissue contact was also ineffective. 2 The magnitude of the photodynamic contraction was dependent upon the concentration of erythrosine, the intensity and wavelength of the incident light, and the presence of oxygen; indirect effects via neurotransmitter release or cyclo‐oxygenase activation were specifically excluded. 3 The photodynamic response was blocked by zero‐[Ca]o and addition of EGTA (1 mM) but not by omission of [Mg]o or a decrease in [Cl]o or [Na]o. D600 (methoxyverapamil) 10−5 M, or a ten fold increase in [Mg]o, to 11.3 mM, partly inhibited the photodynamic contraction at low, but not high, light intensities. 4 These observations are consistent with the following sequence of events: (i) photo‐activation of the erythrosine molecule, (ii) the generation of highly reactive singlet oxygen, (iii) local peroxidation of cell membrane proteolipid, (iv) increased membrane permeability to Ca2+, (v) the influx of Ca2+ and, (vi) muscle contraction. 5 It is concluded that the photodynamic action of erythrosine presents a novel method for modulation of membrane calcium permeability, and hence [Ca]i, not only in smooth muscle but possibly in other cells as well, e.g., secretory, epithelial and myocardial cells.