Potentiation by forskolin of both SNP‐ and ANP‐stimulated cyclic GMP accumulation in porcine isolated palmar lateral vein
1 The aim of this study was to examine the effect of modulation of adenosine 3′:5′‐cyclic monophosphate (cyclic AMP) levels (by using forskolin, a direct activator of adenylyl cyclase, or rolipram, a cyclic AMP selective phosphodiesterase inhibitor) on basal and stimulated guanosine 3′:5′‐cyclic monophosphate (cyclic GMP) levels in the porcine isolated palmer lateral vein by use of a [3H]‐guanine prelabelling technique. 2 Sodium nitroprusside (SNP; 10−5‐10−3M) and atrial natriuretic peptide (ANP; 10−8‐10−6M), produced concentration‐dependent increases in [3H]‐cyclic GMP levels via stimulation of soluble and particulate guanylyl cyclase respectively. The SNP‐stimulated [3H]‐cyclic GMP response peaked after 5 min in the presence and absence of the phosphodiesterase inhibitor, 3‐isobutyl‐1‐methylxanthine (IBMX). 3 In the absence of IBMX, forskolin (3 × 10−5 M) significantly increased [3H]‐cyclic GMP levels to 118.5 ± 8.7% of basal values (P < 0.05, n = 8), and significantly increased both the SNP‐ and ANP‐stimulated [3H]‐cyclic GMP accumulation at all concentrations of SNP and ANP used. For example, effects at the maximal SNP (10−3M) and ANP (10−6M) concentrations were: SNP: 154.7 ± 15.4% of basal; SNP + forskolin: 191.3 ± 14.8% of basal (P < 0.05, n = 4); ANP: 161.4 ± 17.4% of basal; ANP‐ + forskolin: 220.0 ± 20.0% of basal (P < 0.05, n = 4). 4 The cyclic AMP‐selective phosphodiesterase inhibitor, rolipram (10−5 M), had no effect on basal or SNP‐stimulated [3H]‐cyclic GMP levels; however, the combination of forskolin and rolipram produced an increase in the basal (158.7 ± 27.1% of basal) and SNP‐stimulated [3H]‐cyclic GMP accumulation (SNP (10−3M): 165.3 ± 8.7% of basal; SNP + forskolin + rolipram: 510.7 ± 64.8% of basal; P < 0.05, n = 5), greater than either forskolin or rolipram alone. The phosphodiesterase inhibitor, IBMX (10−3 M) significantly raised [3H]‐cyclic GMP levels, and forskolin (3 × 10−5 M) in the presence of IBMX had no significant effect on either basal or SNP‐stimulated [3H]‐cyclic GMP levels (e.g. in the presence of IBMX: SNP (10−3M): 660 ± 90% of basal; SNP + forskolin: 790 ± 86% of basal, n = 3). 5 The data indicate the presence of both soluble and particulate guanylyl cyclase in the porcine isolated palmar lateral vein. The ability of forskolin to potentiate SNP‐ and ANP‐stimulated [3H]‐cyclic GMP accumulation may suggest a cyclic AMP‐cyclic GMP interaction at the level of the phosphodiesterases. Further, the ability of cyclic AMP to influence cyclic GMP levels may indicate that the two nucleotides, as well as having independent mechanisms to induce smooth muscle relaxation, could produce vasodilatation via a common mechanism.