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In the present paper, fabrication, characterization, and physiological applications of a solid-state pH electrode are described. The pH sensing layer was based on an anodic electrodeposited iridium oxide film (AEIROF). Sputtered platinum electrodes (1 mm diameter) fabricated on flexible Kapton films or platinum wires were used as planar or cylindrical supports. Each electrode site was coated with Nation to attenuate the interference of anionic redox species and to protect the electrode surface during in vivo measurements. Performance of the AEIROF was evaluated, for the first time, as a pH electrode and proved to have a slightly super-Nernstian response with slope of -63.5 [+ or -] 2.2 mV/pH unit for both wire and planar sputtered platinum electrodes. Linear pH responses were obtained in the pH range 2-10. The electrodes have a working lifetime of at least 1 month with accuracy of about 0.02 pH unit and fast response time. The electrodes showed very low sensitivities for different species, such as [Na.sup.+], [K.sup.+], [Li.sup.+], N[[H.sub.4].sup.+], [Ca.sup.2+], [Mg.sup.2+], dissolved oxygen, lactate, ascorbate, and urate, which are important for physiological applications. The electrodes were applied in extracellular pH measurements during brief regional ischemia in a swine heart and no-flow ischemia in an isolated rabbit papillary muscle. A first report on extracellular pH, [K.sup.+], and lactate simultaneous measurements during no-flow ischemia using the AEIROF pH electrode and the previously described [K.sup.+] and lactate electrodes is presented as well.

Simultaneous and continuous measurements of extracellular pH, potassium (K+), and lactate (L−) in ischemic rabbit papillary muscle are presented for the first time. Potentiometric pH and K+ sensors and an amperometric lactate biosensor were used. These miniature electrodes were previously developed and individually tested for this purpose. The pH sensor was based on an iridium oxide layer electrodeposited on a planar platinum electrode fabricated on a flexible substrate. The potentiometric K+ sensor was based on a polymeric membrane and valinomycin ionophore. The L− biosensor was based on lactate oxidase and an organic conducting salt polarized at 0.15 V vs Ag/AgCl reference electrode. The utility of this novel analytical system to cardiovascular research was demonstrated by using the system to study the interrelationship of cellular K+ and lactate loss in ischemic myocardium, and the role of extracellular pH and buffer capacity on this relationship. The results indicated: (i) sequential brief episodes of ischemia produced reproducible trends of L−, pH, and K+ changes during the first three episodes, (ii) extracellular L− increased with increasing buffer capacity of extracellular compartment, (iii) the patterns of extracellular L− and K+ changes were not related directly, and (iv) L− transport and lactic acid diffusion were not the primary cause of extracellular acidosis during ischemia. [Copyright &y& Elsevier]