Cation channels involved in cardiac ion homeostasis include a magnesium inhibited, transient receptor potential melastatin 7 (TRPM7)-like channel. To understand the regulation of cardiac TRPM7-like channels, we used the whole-cell voltage-clamp technique in pig isolated ventricular myocytes and investigated the role of G proteins. The TRPM7-like current, which can be induced by intracellular dialysis with low Mg²⁺ concentration, remained at 96.8  1.1% of its maximum at 60 min after patch rupture when the intracellular solution contained GTP (100 mM). Substitution of GTP by its non-hydrolyzable analog GTPgS (100 mM) decreased the current to 7.5  9.3% (P < 0.05) within the same period, whereas substitution by GDPbS (300 mM) failed to induce run-down. Under dialysis with GTPgS, the inhibition of phospholipase C activity by edelfosine (ET-18-OCH₃, 10 mM) or the intracellular addition of exogenous phosphatidylinositol 4,5 bisphosphate (PIP₂, 20 mM) prevented run-down. Under the same conditions, extracellular application of the agonists carbachol (100 mM) and phenylephrine (40 mM), known to be coupled to phosphotidylinositol metabolism, accelerated the rate of run-down (decay t_1/2: from 14.2  2.0 min to 6.8  0.9 min; P < 0.05). These results suggest that G proteins modulate cardiac TRPM7-like channels via a mechanism linked to phospholipid metabolism in a manner consistent with our previous findings showing the need for PIP₂ in maintaining channel activity (Am J Physiol Cell Physiol 2006; 291: C627-35).