In vivo detection of a pH-sensitive nitroxide in the rat stomach by low-field ESR-based techniques

A study was made of the in vivo detectability of a pH-sensitive, imidazolidine spin probe, and the efficacy of low-frequency electron spin resonance (ESR)-based techniques for pH measurement in vitro and in vivo in rats. The techniques used were longitudinally-detected ESR (LODESR) and field-cycled dynamic nuclear polarization (FC-DNP) for in vitro and in vivo measurements, and radiofrequency (RF)- and X-band ESR for comparisons in vitro. The spin probe was hexamethyl imidazolidine (HMI) with a pK of 4.6. All techniques detected HMI. Detection by FC-DNP implies coupling between the free radical and solvent water spins. Separations between the three spectral lines of the nitroxide radical, relative to measurement frequency, were consistent with theory. The overall spectrum width from unprotonated HMI (pH > pK) was greater than that from protonated agent (pH < pK). This was observed in vitro and in vivo. Longer-term studies showed that HMI is detectable and has the same spectral width (i.e., is at the same pH) up to 2 hr after gavage into the stomach, although the magnitude of the signal decreases rapidly during the first hour. These findings demonstrate the suitability of LODESR and FC-DNP for monitoring HMI and measuring pH in vivo. These techniques would be useful for monitoring disease and drug pharmacology in the living system.